Sound advice from Walt Mossberg —
I would never run a Windows PC without a good, constantly updated antivirus program, period. Unless your ISP is supplying software like this that resides on your PC, rather than on its servers, I urge you to hasten to a store or a Web site and buy an antivirus program immediately. Install it right away, and make certain you keep it up to date. Do not procrastinate on this. There is an international group of vandals and criminals whose sole purpose in life, every day, is to unleash viruses and other malicious programs onto your PC.
The only exception might be if you own a Macintosh. Windows viruses can’t run on the Mac, and I know of no viruses that do run on the Mac’s OS X operating system and that have spread to the public. I still advise running an antivirus program on a Mac, just in case. But it’s a much less urgent matter.