Use and reccomend Kaspersky Antivirus or Internet security myself.
You can get a free 30day trial at www.kaspersky.com and it's around £20-£30 to buy for a year after that.
Been using the Internet security and Windows firewall for a couple of years now with no issues at all.
However, whichever antivirus you end up with the best defence is being savvy and not going to dodgy sites or downloading junk.
Works on both 32 and 64bit versions of windows 7.
If you decide on something else then for the love of god make sure it's not McAfee or Norton.