Sam's got some great points there, and in terms of web browsers I'd recommend Opera!
However, you do say your PC is slow. I'd recommend you'd do this:
* run a spyware app! http://www.hitmanpro.nl is great. It's got 5 or 6 different spyware scanners in the same software.
once that's done'
*do a defrag of your system disc/ partition. Perfectdisk is the software to search for there. It's a very neat and very efficient defragger and will do the job a lot quicker than the Windows one ( unless you haven't defragged in ages, if ever

)
Those two things should be done at least once a week. That'll keep your PC running quick, unless of course, you've got some hardware probs aswell. ( Make sure your HDD isn't getting full also; that will slow it down loads)
Hope you get it sorted!
Cc