Virus Scan Windows 8 Bootcamp Partition from OS X?
Two problems:


I am not sure of any good Mac solution that will have the coverage of Windows viruses that Windows tools such as Security Essentials do. ClamAV may have pretty good signatures, but I can't vouch for whether it does or does not have a comparable scan and fix ability to a commercial product. Mac virus scan programs may be better, but again, they would probably have more focus on Mac viruses.
By default, NTFS partitions are not writable on Mac OS X. You therefore could detect, but not fix or remove, viruses from your Windows partition. The only way around this is to purchase a commercial product such as Paragon NTFS, or go through some interesting challenges to get open-source drivers to mount the partition read-write.


If you use Microsoft's offline version of Security essentials (http://windows.microsoft.com/en-us/windows/what-is-windows-defender-offline) and your Mac is reasonably new, you should be able to make a bootable USB drive that will work - just insert it and hold down Option as you power on your Mac.