An interesting blog by Brian Krebs:

"Security experts have long maintained that running two different anti-virus products on the same Windows machine is asking for trouble, because the programs inevitably will compete for resources and slow down or even crash the host PC.

... But an upstart anti-virus company called Immunet Protect is hoping Windows users shrug off this conventional wisdom and embrace the dual anti-virus approach. Indeed, the company’s free product works largely by sharing data about virus detections from other anti-virus products already resident on the PCs of the Immunet user community.

... I’ve been running Immunet in tandem with Kaspersky Internet Security 2010 for the past three months, and have haven’t noticed any impact on system resources or stability issues. Immunet’s creators are especially proud of that last aspect of the program, and say it’s due to the fact that the program does most of its scanning and operations “in-the-cloud,” – that is, not on the user’s system. "

Full read: http://krebsonsecurity.com/2010/04/immunet-a-second-opinion-worth-a-second-look/

http://www.immunet.com/

Haven't tried this yet, but Krebs does know his security stuff.