antivirus free or paid
Other than the 64 bit situation I agree with Bobo, that AVG is great for a free version. There are other great paid versions, but not all paid software is better than free software. For example I paid approximately 39.95 and 29.95 for McAfee's firewall and antivirus protection and a re-up fee at the end of the year for approx 25-30 each, and it found little or no problems yet I still had some. I put Zone Alarm on by Zone Labs which is only 29.95 for both antivirus and Firewall with only a $20 re-up fee yearly and it found the worm that McAfee could not. I had previously tried Norton's as well and with approx the same price and same results as McAfee. To pay more and get less coverage seems ridiculous and both McAfee and Norton's also have a long mean boot time, which makes them big resource hogs. I would use anything but McAfee or Nortons.
Also I tried Trend Micro's PC cillin and it also could not detect the worm. I went through 4-5 software companies before I found one that could fix the problem. Trend Micro was rated number 1 in 2005 by Consumer Reports and by Fall of 2006 it has fallen to almost 9th or 10th place.