keep in mind that many viruses (virii?) are not actually destructive to your system... at one time virus' would be so malicious as to format your C:\ drive or delete all your .jpg files... sometimes with a cute message telling you what they were doing... they've now realized there's no money in that. The big money now is in creating "zombie" networks to send out spam or use the combined power to brute force break passwords, or keyloggers to steal passwords... many smart enough to run in background or at night where you don't even realize they are running - with tactics like that, how could you tell you were infected or not WITHOUT AV software? Just something to think about.