With Microsoft neglecting the once excellent Security Essentials, one is always on the hunt for a good replacement. It's hard to beat free though, unless of course the option you're considering is also free. Bitdefender is currently offering its AV software for this exact price — well, for six months at least — providing you with the chance to give it a solid try.
"Henryqld" over at OzBargain spotted the limited-time deal today, which is good for the next 10 or so hours. All you have to do is provide your name and email address, after which you'll probably have to deal with a bit of spam.
The only problem with the deal (and the point of it really), is that if you do fall in love with it, you'll have to find the cash to pay for a proper subscription after the half-year period is over. Either that, or just master the art of tracking down and chaining free offers.