you prob have some corrupt sectors in your HD, well happens really rarely or your HD is set incorrectly. i dont support viruses or anti virus, for viruses its somewhat rare to get a virus i think unless you have enemies posing as friends wanting to send you a file called "mymomnaked.avi.exe", seriously i download via torrent/usenet (prob 30+gb a month misc files) and havnt had a virus in like 5 years but i do see people who come to me to repair their pc with unbelievable amount of virus/spyware on their pc.
anti virus also i think is useless, cost money, uses crazy amount of resource (i dont care if you have a 90ghz cpu, you will still feel AV affects by HD access it does) and if you use your PC for games i say get something without active scanners and scan frequently.
alternatively, heres what i do use free AV built into email service etc, send yourself a email, exe file something not huge, like 500k-1mb, and scan it like that (yahoo, hotmail, prob buncha others) also you can use free virus scanners like ones www.mcafee.com has
*Edit*
oh great way to scan suspicious files using email and mail it to yourself before you open it