AVAST actually rocks on my Vista powered notebook. One thing I like about it is that it updates frequently and I haven't had any problems or system performance issues.
AVG - I tried it but someone told me (here on CE) that it only checks for the viruses/trojans but doesn't remove them. AVAST had great reviews on other forums and I decided to switch to it.
I'm not quite sure about QuickHeal. They do advertise a lot though
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