Do you have anti-virus protection? Are you using any spyware/malware removal tools? If not, do so. Since you've been a lurker here for awhile, you've probably seen the usual recommended programs for this. And, I imagine the school you attend may have some info about avoiding computer problems from online exposure. Am I right? Nevertheless, if you need more info on such tools, post here again.
Also, do you periodically run Disk Cleanup? And, delete temporary internet files?
What OS are you running? If Win XP, do you have SP2 installed?