I used the AVG free version for a good five years and it did its job great, except for one time when I got badly infected by a nasty rootkit and ended up having to reformat. Still though, no program is fail-proof. I just recently switched to AntiVir and I'm liking it a lot so far (especially the fact that it has a rootkit scanner

) I think both are probably good enough for the average person as far as free programs go.
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There really is no point in a full scan everyday.
There is if you want to keep your machine clean! These days it's so easy to get infected without even knowing it, I wouldn't go more than a day or two without a scan, and I still usually scan it with multiple different programs weekly. Why not just schedule it to run when you go to bed or while you're at work or something? Most programs even have an option to shut the computer off when the scan finishes if you don't like leaving your PC on.