Last week Microsoft released version 2.0 of Security Essentials, its free antivirus scanner for Windows. If you're currently running version 1, you can get the latest release from http://www.microsoft.com/downloads/en/details.aspx?FamilyID=e1605e70-9649-4a87-8532-33d813687a7f in order to benefit from additional features and advanced detection technology.
It runs on Windows XP and above, and is available in 32- and 64-bit versions. The download is around 9 MB.
If you don't currently have any antivirus software on your PC, it's vital that you do. If not for yourself, then for the sake of any friend, relative or colleage to whom you risk sending an infected file.
I've actually been using Microsoft Security Essentials v1 on one of my own PCs for the past few months, and have found it to be very reliable and stable. I shall upgrade to version 2 over Christmas and let you know if I find any shortcomings.
Security Essentials is free, although Microsoft does point out that you need to be running "genuine Windows" in order to be able to download it. While I'm not about to condone software piracy, I do think that Microsoft should make an exception here, and allow MSE to be used even by those with illegal copies of Windows. It would help to reduce the spread of virus-infected files to legitimate users, which would help to prove that Microsoft really does take security seriously.