I use Mcafee which comes free for airtel broadband customers never had a problem so far...Most of the major ISP's do give free antivirus so check with your net provider..
go for Avira AntiVir its free and effective or go for the free version of Kaspersky the AOL free antivirus called Active Virus Shield also free and its KASPERSKY.
i vote for kasperksy, i have been using it from a long time, and it is not eating my sys resources, even though i have a "not so good" machine. all games run fine, no hassles while normal working either.
PS: you can install kaspersky only with antivirus features, and its even less heavy
Tried kaspersky but teh real time protection kept taking too much time scanning files etc when i was accessing folders n kept locking files unnecessarily so ditched it.
Too many similar posts already in the forums!! Anyways, Kaspersky is the best and version 7 has some advanced on-demand scanning for improving the heuristics... Nice sandbox'd technique... I think in the next antivirus roundup on the net we'll find Kaspersky 7 leads!!
PS: Kaspersky is really NOT RESOURCE HUNGRY these days and should run perfectly fine with 1GB RAM, even on Vista... The initial runtime file scanning makes a little HDD effort, but then if u have the option "scan only changed files" enabled, it is pretty smooth on the system!!