Free, AntiVirus for Windows, Free
Individual Windows users concerned with cost have multiple choices for free anti-virus software. All of these publishers want to impress you with how good their systems are, to encourage you to buy their products or services, but that seems a reasonable trade IMHO. Reviews of some of these may be found here and there.
AOL Active Virus Shield (requires you accept spamvertising, which can later be turned off).
AVG anti-virus and anti-spyware
Antidote SuperLite (scanning only)