Everything Richard says, +42 ....You don't say which version of Windows.
My recommendations (all free - I don't believe in paying when there is now an excellent free alternative):
Windows XP: Microsoft Security Essentials
Windows 7: Microsoft Security Essentials
Windows 8: Windows Defender (comes built in to Windows 8).
Please note that Windows Defender in Windows 8 is NOT the same as the Windows Defender in Windows 7, which is not an anti-virus tool.
In addition I would install Malwarebytes to do occasional scans with.
Makes a lot of sense to me as I sometimes share the risky surfing habits, some would say cavalier attitude as I am pretty confident I can deal with any issues that may arise - of course it could also be seen as overconfident and dumb :wink:. Not had anything too bad happen with MSE and certainly no virus notifications but have had a couple of instances when MSE suddenly stopped working and could not be restarted without uninstall and reinstall - which I did find mildly disturbing as I could not reliably identify why.... I am a somewhat risky websurfer and sometimes recklessly install programs to test when I should be using a VM instead. Too many times I found MSE letting me know I had a virus after the fact, which is pointless. It then goes on to say it can't clean it. It needs to prevent it for it to be useful at all.
Good browsing and program installing habits are your best defense against malware these days, since nearly all AV solutions have a max 60% detection rate. With that said, I have had the most success with Avast free. It also has a boot scan which so few others do. Malwarebytes is a good utility to remove trojans (or prevent them if you pay for that version), but its not an antivirus solution.
Something to consider using on risky software and websites (open your browser in it): www.sandboxie.com
This site I frequent when looking for free apps that are not bundled with malware: http://www.techsupportalert.com/