There are quite a few good free antivirus programs out there. The one I use is Avast Antivirus. Excellent detection rates, oodles of features, and a low amount of false positives. You can download it from http://www.avast.com/free-antivirus-download.
Another good one, although not as full featured as Avast, is Avira, which can be downloaded from http://www.free-av.com/en/download/1...antivirus.html.
Finally, Avg antivirus is the proggy I used to use, but as they've gone to an ever increasing array of paid versions, the free version has had some functionality stripped out of it. I also had problems with false positives. You can find Avg at http://free.avg.com/us-en/download-avg-anti-virus-free.
If your wife is going to be going on the net, I would recommend in addition to the antivirus program a good firewall, such as Comodo, at http://personalfirewall.comodo.com/, a behaviorial shield/antispyware program such as Threatfire, at http://www.threatfire.com/ and a good malware/anti-trojan scanner, such as A Squared, at http://www.emsisoft.com/en/software/free/
A final layer of protection that you can use is a program called Process Guard, which blocks attempts to alter the process stack of running programs in windows. Its quite powerful (perhaps overly so), but can significantly diminish the functionality of windows as it takes awhile to learn what are the system's normal processes. It can be found at: http://www.diamondcs.com.au/processguard/ I do not use it due to the extra hassles it can create.
I've found that no one program does the trick, but a multi-layered, multi-program defense has kept my system relatively infection/problem free. Good luck and keep those maps of yours coming