As a further update to the above.
I recommend the FREE AV system from
www.grisoft.com. It's at least as good as payware stuff like McAfee and Norton from my experience.
Today I updated mine to the 31st October release. When I rebotted it detected a trojan in the system area and when I ran a full scan it picked up and removed 6 (YES SIX!!) trojan/virus files.
Anyone who is running any kind of Internet connection without a system like this is looking for trouble.
BTW AV software MUST be updated at least every 3-7 days. It's no good saying you've got Norton or McAfee that came with your system 18 months ago - it is now worse than useless.