Here’s a web site that may come in handy: Microsoft’s new Safety Center lets you run free scans for viruses and other malicious software. It also includes maintenance tools for defragging and cleaning up your disk.
If a virus gets onto your system somehow – you let your subscription lapse, or your virus definitions were out of date, for example – you may find that the virus prevents you from using whatever antivirus software was installed. Online scans can sometimes get you out of a jam! There are similar free online scans from Trend Micro, Panda, and others.