I have listed the most obvious reasons why a computer may be running slow, some of which you may have already tried but for completeness they are mentioned here.
As a reminder, if you are using a laptop it will be running slower on the battery as opposed to running on the mains power.
First of all make sure you do not have any viruses or spyware. Run your anti virus and anti spyware programs in order to clear out any beasts. Spyware is becoming more and more of a problem because some spyware is extremely difficult to identify and even more difficult to remove.
If you do not have any protection these applications are exceptional and available for free download:
Remove junk & temporary Internet files that may be cluttering your system by running Ccleaner – free for download from:http://filehippo.com/download_ccleaner/
You may have too many programs kicking off at start up. These two free downloads allow you to easily take control what applications run at start up. The applications are not deleted they are just prevented from running at start up.http://www.glarysoft.com/quick-startuphttp://www.mlin.net/StartupCPL.shtml
Your hard disk may be getting full. You need at least 15% of free space for it to operate efficiently. Go to Control Panel > Add/Remove Programs & remove any applications you no longer use. How about removing a lot of the videos, music and photos that are filling up your hard drive?
Your hard disk(s) may be fragmented. Windows has a built in defragmentation program but you can download a quicker third party program for free:http://www.auslogics.com/en/software
Your registry may be cluttered or corrupt. Ccleaner mentioned earlier also has a registry cleaner and so I have linked it again but how about downloading another very good free registry cleaner:http://www.eusing.comhttp://filehippo.com/download_ccleaner/http://www.free-windows-registry-cleaner…
Finally, after you have done all the above may need to upgrade your computers memory. Normally Windows XP is quite happy with 1GB but for Windows Vista I suggest upgrading to 2GB to get equivalent speed and efficiency. In order that you get the right type of RAM for your system do a free scan at Crucial to determine your RAM.http://www.crucial.com/
Now that you have attempted to improve performance what about some maintenance to hopefully prevent overheating? Clean all the vents on your computer with an air duster (can of compressed air). If a laptop, operate it on a hard surface so that air can flow freely around it. For a desktop, open it up and clean inside with the air duster.