I'm a NOD32 user here.
I would say that buying a good AV solution is probably the best investment you can make. Free is good as a temporary fix, but remember the free versions normally don't auto-update & run a few hours behind as well.
Avoid Norton like the plague if you actually care about using your PC for anything other than staring at an hour glass. The same goes for F-Secure. We've got the latter at work & boy do I wish we hadn't. I never thought I'd see a core2 hit 100% usage for 5 minutes when booting, but that's what F-Secure does for you.
For an alternative, try Kaspersky. I've seen some very favourable reviews recently; especially from gamers.
Another useful utility is Malwarebytes Anti-Malware. It saved me from thinking I would have to wipe my install when I got the Virtumonde infection. Nothing else saw it until MAM had cleaned up a bit.
Filehippo is a great site & the free update checker is a nice feature.
The main thing to remember with any utils you download/buy, is that if they work for you, then stick with them. As you can tell, I detest Norton & F-Secure, but a guy in our IT recommends Norton to all his mates. Mind you, he plays games on an X-Box & uses his PC for e-mail only.