Firstly, you deserve a medal for keeping a PC alive and running for 10 years. that's a fair effort. I think my best effort is about 3 years.
Without taking a good look at your computer it's hard to diagnose but ideally you'd want to start with the cheapest options first with spending money being the last resort.
it could be a virus or nasty software. - running scanners / cleaners should turn something up it that is the case.
it could just be a simple glitch with your OS - in which case you can probably do a 'refresh' by inserting your OS installation disc - similar to reinstalling but should keep your other data intact.
Your system hard drive might be damaged. I had a similar issue recently and that turned out to be the culprit.
Scan your drive/s, run virus / malware checking software.
Try running Linux from a cd. similar issues?
Back up your files [for piece of mind] and try performing an OS refresh?
failing those, reformat and re-install your OS. see if the problem persists.
NEVER smack your PC. This causes far more problems than it solves in the long run.
it could be some other minor thing that I/we haven't covered. If you try some of those it might narrow things down if it doesn't solve the issue altogether..