The older your computer is, the more likely that your registry has encountered some flaws.
Registry Mechanic is a great program for cleaning up the registry, but you need to download it from a file-sharing program.
Another very good registry fixer is Registry Patrol, which you can run for free from their website.
The following link describes how registry problems develop and also provides a link to the Registry Patrol website:
http://www.asknerd.net/registryfix.html?OVRAW=registry%20editor&OVKEY=registry%20error%20fix&OVMTC=advanced&OVADID=4140372011&OVKWID=11038584011
Beyond this, you might want to alt-control-delete to bring up the list of processes that you have running on your comp. See if your resources are being used up by some unwanted programs running in the background. Either way, a registry fixer should certainly help for speed and stability.
GL with it
Comp problems are a bitch. I recently had a nasty virus that took quite an effort to get rid of.
Fixed it, though.
Knock on wood.