I have Windows 7 in Bootcamp but it includes Microsoft Security Essentials which I assume Windows 8 probably has as well. I was told that unlike earlier MS stuff that this was good software that would take care of the issue. So that is what I have been using since April 2010.
So I would suggest looking into what security software is included w Windows 8 & see whether it would do the trick. Of course I'm not sure if they include other anti-malware software w it. When I was on XP before coming back to Mac, I had anti virus, anti-malware & a software firewall program. But have just used Microsoft Security Essentials w Windows 7.
In addition, I'm about to upgrade from Parallels 5 to Parallels 8 (after finishing prep such as backup) so that I can use it w Mountain Lion that I purchased & am waiting to install until after I get the Parallels update done since 5 doesn't work w Mountain Lion. But in Parallels 5 it offered a free 1 year subscription to Kaspersky antivirus or it may have been more than anti-virus.
So that is 2 possible free options to protect Windows to check out.