OK, Try a virus scanner. you can get a free one from www.grisift.com
a fire wall is good for guarding against port scanning and worm attacks. you can get a free one from www.agnitum.com
Spyware is the bane of the windows users life, you can get a free spyware remover/blocker that even prevents programs changing the registry without your sayso if you like from security.kolla.de
If you need to remove programs that arn't registered as either you try use hijackthis from www.spywareinfo.com/~merijn/downloads.html
-but the test results from this can be quite hard to understand.