Do a Virus scan and say that...
Anti Virus programs are there to HELP your PC, I'm running Windows XP Pro SP2 (Massive Improvement on Unpatched, a lot less bugs and stuff) with NOD 32 Anti Virus, does me well.
If you don't run any antivirus programs, your an idiot...
Your PC is a lot safer behind an Anti Virus program and some Anti Spyware/Adware/Malware programs like Spybot Search and Destroy, and Ad Aware
Just don't get McAfee.
I would SERIOUSLY recommend getting AntiVirus, there are so many things out there on the internet that can really mess your PC up.