I would say Avast ant-virus, or Avira Antivir the free home versions are at
filehippo.com/software/antivi…

BUT before you install it make sure that you have removed ANY OTHER anti-virus program, because 2 of them can conflict and slow down the computer badly, or freeze it up.

To uninstall a program in Vista or Windows 7, click the round windows flag (start), then click on control panel, find "Programs and Features" and click on it, it may take a few seconds to fill in the list.

If you've got XP, it is in Start==> Control panel==> "Add and remove programs"

You will want to remove anything Norton, Symantec, Kapersky, or McAfee, there might be others, but those are a few examples of the pre-installed junk anti-virus programs.

DON'T even use AVG its worthless (IMHO).

avira is the best free antivirus. i also have avast installed (which is still better than avg), but i have avira doing the constant monitoring and i just use avast to manually scan files that i think are suspicious. so far, avast has never caught anything that avira didn't first.

free-av.com/

Well the quality of antivirus software has almost nothing to do with what kind of computer you have but if you're just asking for a good program I'd recommend AVG Free.download.cnet.com/AVG-Anti-Virus-…

I'd get iolo
I already have system Mechanic and if the grade is as good, i'll be very happy

If you are looking for some free security program then simply go for AVG antivirus security software...
If you are looking for paid then McAfee premium is really good one!!

Your best option would be to use Linux and not have to worry about having anti-virus.