There are plenty of reasons why your computer might be slow.

Make sure you firstly check if your windows registry is free from damage

Your computer registry is the powerhouse of your computer. If your windows registry is not free from damage and well organized, your system will become unbelievably slow and is likely to start freezing, and crashing. You should regularly clean your windows registry with and effective registry cleaning product to ensure that you computer is running fast and your registry is performing optimally.

Here are some possible causes: Your hard-disk is fragmented. Your do not have enough RAM. You are running low (or set too high) on virtual memory. Your windows registry contains many invalid entries. Try http://www.registryreviewgroup.net for Registry Cleaner software reviews Your system files became corrupt. You enabled additional visual effects. Your hard disk contains bad sectors. Your CPU is overheating. Your network or Internet connection is slow. Your computer is running too many programs in the background. Your hardrive is not partitioned properly. your operating system drive (C:) free space will be too low. You have a Trojan/Virus/Malware/AdWare/Spyware using resources. You have Norton Anti Virus installed. Too may startup programs & Startup services Here are some fixes you might want to try:

Disk Fragmentation:

Defragment your hard drive with the windows defragmentation tool, this might take a while especially if this is your first time doing it but is well worth it. You can do this by going into your start menu-> programs-> accessories-> >-system tools-< and by selecting “Disk Defragmenter”.

WHY?, because when windows installs or delete files from your hard drive it doesn’t always put it back to its place to save speed or simply because the value of the file moved up or down. But what is the value?, the value is set when you use a program more than another thus making that program more important for your operating system and so windows will move that program to the beginning of your hard drive so that it can load it faster (eg: the beginning of the drive is faster than the end).RAM (Random Access Memory) Issues:

How can i free some memory?, You could unload your animated background and use a static one. You could also disable some windows enhancements by going to control panel-> system-> advanced and then under performance-> settings and disable some options. You could also select a lower color setting in your video options by right clicking onto your desktop and then by selecting properties. Or if all else fails, buy some, they are really cheap now a days and is always worth it but don’t go overboard if you are using 32bit Windows XP version because it will not detect more than 3 Gigabyte of memory, for this you will have to use the 64bit version of XP and enable memory remapping through your bios, so 3GB max=32bit and 3GB+ =64bit!.Virtual Memory Issues:

What is virtual memory and what is it used for?, Virtual memory is used primarily by windows kernel (your operating system) and/or multimedia programs such as video converters/editors, audio or other demanding programs/applications. Since you will never have enough memory to load everything into your RAM, Windows will create a space to use onto your hard drive just like your physical memory so that you can play your games while other programs still runs in the background, another way to put it, its like your RAM but on your hard drive!.

Low or High, why does it slows down?, When set too low you simply running out of memory, when set too high your computer slows down because your hard drive is much slower than your RAM, in fact your memory is about 1000X +++ faster than your hard drive so when you allow windows to use too much virtual memory (this happens when you do not have much ram or by setting it yourself) it takes much longer to load your programs.

What should i do?, you could buy more ram to allow windows to use less virtual mem or free up some disk space because this might be the reason you are low on virtual memory since widows allocate this memory every time you boot up (restart) so if you used more disk space before you restarted, windows will use what is left of your hard drive for virtual memory. In case it is too high, you could change the setting back to leting windows take care of your virtual memory usage.Registry Issues:

When you uninstall programs, delete, view, cache files the windows registry will keep some of the information either at the will of the program or simply because the program was deleted manually thus leaving all the information stuck into your registry and so windows will have to load all of this useless information when it starts thus making it slower than it usually is. TRY A REGISTRY CLEANER SOFTWARE

To rectify this problem you can edit your registry (BE CAREFUL!!!!) manually if you know the keys left back by the program or to be on the safe side and do a much better job you could use a Registry Cleaner  to do the job.

Try http://www.registryreviewgroup.netfor registry cleaner ratings

File Corruption Issues:

File corruption primarily occurs when the computer isn’t shut down properly. Other reasons could be that your HDD (hard disk drive) is over heating, the acoustic setting is too high or simply needs to be changed. This could also happen if your PC is Overclocked.

To fix corruption just right click onto your primary drive (usually C) and click on properties-> Tools then in the error checking section click check now and answer yes to the query and then reboot. If it can not be fixed then you have a “Bad Block”, a bad block is actual physical damage to the drive and should be changed immediately.

To cool down your hdd’s, if you have more than one and if possible, leave a space between each one or buy an heat pipe cooler from your fav computer store.

 Please refer to the “RAM Issues” for help.Bad Sectors Issues:

Please refer to the “File Corruption Issue” for help.CPU (Central Processing Unit) over heating issues:

If your processor is overheating (you can determine this through bios or motherboard utilities such as Asus Probe if you have an asus motherboard) you will have to clean your heat sink and if possible change your fan because cleaning your fan will just put dirt inside where you bearing resides and make noise while the fan is turning. If this doesn’t work or while your at it just change your thermal paste. After a while thermal paste dries up and stops conducting the heat to your heat sink so a change is needed.

Internet Connection Issues:

I will only give you references since the cause could be a million different things.
Your problem could be:

ISP problem

Line quality problem

Windows settings such as MTU and Receive Window

Router issue

Firewall issue

Hijacker problem

The programs you are using

Without further details, i can not remedy your problem.Background Startup Program Issues:

This basically becomes a ram issue, please refer to “RAM Issues” for help.

Partitioning Issues:

Under the Parallel ATA architecture (ATA or UATA), a 250GB hard drive must be partitioned in at lease 2 parts and under Serial ATA architecture (SATA), a 500GB hard drive must be partitioned in at lease 2 parts.

Why?, because when a 250GB ATA and a 500GB SATA hard drive gets half full the heads inside the hdd are doing twice the work because it has to search through 250GB and 500GB of data but when the drive is split in 2 the partition sets boundaries so the heads only has to search through half the disk.

How?, you can do this at windows installation or with a program called “Partition Magic” but be careful because if you partition your disk you will automatically loose all data so back up first and have your windows handy!.