If your PC is running slow...because of an infection

This is completely different from the effects of fragmentation. The cause of slowing down is not physical. It is due to the activity of a possibly malicious program trying to get CPU time to do nasty stuff. It could be either spyware or virus causing this. There are several ways you can work this out.
1) Download Spyware Blaster from http://majorgeeks.com/SpywareBlaster_d2859.html"
2) Install it and update its database regularly. It will maintain a list of 10000 plus nasties and turn off all registry keys in windows that belong to them. Even if you have the bugs, they won't do any harm!
3) Download bitdefender version 10 (free) from www.bitdefender.com and install it. After updating its list of infections, this will clean out many infectious things from your copy of Windows. If you still have problems after that, it may be wise to have it professionally serviced.