What Reefer said …
But check for background activity …
It's probable that your computer will have programs running in the background. This limits the memory available and the processor might be doing unnecessary work. Programs will often set the Load on start-up feature automatically when installing so de-select this option during the installation to take control.
Check the icons in your computer's ‘system tray’
(normally bottom right hand corner of the screen on a PC) to see background programs. You will find that your printer, audio software and numerous other bits are sitting waiting to spring into action. This is fine if you have a powerful machine but annoying on an old machine or a laptop. A quick fix is to hover over the button and right-click to select the Close option.
If you don't want background programs to start next time you boot: Find the program and either configure it not to run …
You will have to select the programme to run it when you want it … or remove it from Programs/Start-up so it won’t automatically start. This should significantly speed Windows start times and overall efficiency. If it is completely redundant delete it from your system.
Some file systems work better than others for large disk partitions. If the machine runs Windows 2000, or Windows XP, you should use the NTFS file system for best performance. If you are not using the NTFS file system, you may be able to improve performance by moving files and folders out of the root directory.
With FAT partitions, you may notice a big slowdown in system performance after running scandisk because a large number of
.chk files are placed in the root directory. Users sometimes fill their root directories by making it the default file storage location. Move as many files and folders as possible out of the root directory, and performance should improve significantly. Converting from FAT to NTFS is an option that needs to be done via a ‘clean install’ of the OS.
Modern processors generate a lot of heat. That’s why all processors require some sort of cooling element, typically a fan of some type. When the processor temperature goes over spec, the system can slow down or run erratically. This is especially true for Intel P4 CPUs
The processor fan may fail for several reasons:
Dust is preventing the fan from spinning smoothly.
The fan motor has failed.
The fan bearings are loose and “jiggling.”
Often you can tell if there is a fan problem by listening and/or touching the computer. A fan that has loose bearings starts jiggling and vibrates, making an identifiable noise. You may barely notice the noise at first, and it’s common for even the experienced computer professional to overlook this change. But as time goes by, the sounds and vibrations will become so prominent that you’ll change the fan out just to stop the racket.
You don’t always need to replace the fan. If it is covered with dust, you can often blow away the dust, I use a vacuum cleaner.
(very carefully) Note that even though you might get the fan running again, its life span has likely been reduced because of the overwork. You should keep an extra fan in reserve in case of failure, if possible.
Go to the manufacturer of your hard drive, download their diagnostic tools and check your HD
HARDWARE FIRST: When troubleshooting a system slowdown, you should always look for potential hardware problems first. Then, investigate the common software problems. If you use a systematic troubleshooting plan, you should be able to improve the performance of most computers suffering from system slowdown.
i turned down the fan on my PSU because it was loud, it wasnt till reading your post i remembered 
