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).
avast!
Avira PE
AVG anti-virus and anti-spyware
ClamWin
Comodo
Antidote SuperLite (scanning only)