Everyone is right in their advice. But the tip off is you saying you install and unstall programs. The leads to a cluttered up reg. The system is still looking at those reg entries and has to keep up with them even if the program is gone. Most uninstallers are junk and leave behind a mess in the reg. I would suggest getting a cleaner like crap cleaner. That's its name. But despite that it works and is safe to use. You get it from www.ccleaner.com
When you go to use it turn on all the functions and run it untill it sees nothing. It may take several passes if you have a really junked up drive. Oh yeah if you have any cookie based passwords in the system it will clean them out also. A minor inconveneance.
Now as for the rest get spybot and adaware. Update them BEFORE you run them. Then get on a routine about running all these programs. Makes things run smoother and quicker.