I tired several AV Programs and eventually settled on eSet-NOD32. I've been told by what I consider an informed, reliable source that it's less of a resource hog. Most of the testing I did was under XP-Pro while building a new system for Vista-P-64-Retail.
The new system has had NOD32 on it from the start. (yeah it's only 32bit but so are many other programs that work just peachy with a 64bit OS) Like most Windows versions there is a good deal of activity that goes on after the desktop shows up but before the system is ready for use. I noticed it took longer after I loaded eSet. Hmmm...
OK, so I put Process Explorer in my startup folder and watched the post log on activity. (as well as the CPU useage from all processes while doing what I normally do with the PC) ESet was scanning each module/file as it was accessed, i.e. doing it's R/T file scans & etc. IMHO, eSet's overhead is acceptable and it seems to perform well.
The free edition of AVG didn't perform nearly as well and the catch was that it's nagware. They want you to buy a 34.95 (and up) version and gives you popup windows after evry scan to "inform you" of what you're missing. The biggest red-flag for me was no root-kit protection. You can find a product comparision at grisoft.com/ww.home-and-office-security-comparison
Avast Home Edition (free) was similar in performance to AVG. The thing I didn't like about it most was limitations on what you can do with scheduled and ad-hoc scans. As best I recall for example I wasn't able to scan a specific directory, only an enitre drive. The comarison with the pro edition is at avast.com/eng/avast-compare-home-professional
The best source I've found for un-biased reviews of effectiveness is at av-comparitives.org. A number of AV programs and industry publications quote this site as one of their references.
Good luck and I hope this helps.
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