Alwils avast4 ! click here This has the best results on the Virus Bulletin tests click here for any free anti virus. The scan scheduler is disabled, but you can set it to scan during screensaver, updates are incremental and fully automatic. Highly recommended.
Grisofts AVG 6.0 click here currently has Virus bulletin 100% click here but pass history not as good as avast. Used to be included on PCA cover disks, so is most common option used here, has equally vociferous supporters and detractors. Current commercial version is AVG 7.0. In my opinion 6.0 is beginning to creak a bit, but still a sound product.
H+BEDVs Antivir click here has never passed the Virus bulletin tests click here. No limitations to functionality and has support via Antivir forum. Not the easiest program to set up, with slow scan and large update downloads. Functional but not my first choice.
Microworlds Free eScan Antivirus toolkit click here an on demand scanner only, but also picks up spyware, dialers keyloggers and other bits, so may be of interest as a backup