How Macs run Windows programs:
Before Apple switched to Intel CPUs, they did it with an emulator. Now that the newer Apples use the same CPUs as Windows boxes, it's still an emulator, but it doesn't have to work as hard and doesn't have to translate every single CPU instruction. The answer is actually a lot more technical than that, but that's the shorthand.
Intel based Macs don't run emulators to run Windows, or any other OS. When booting from a clean start into Windows, it is no different then booting any other Intel based PC, with the small exception that it uses EFI to boot. Apple isn't unique in this regard, and Gateway and others have shipped consumer machines with EFI that boot Windows. Apple offers something called Boot Camp, but all it does is resize your OS X install to allow Windows to fit on the disc, and provides drivers for the Mac hardware under Windows. No voodoo magic here in booting Windows, or Linux or whatever other Intel CPU compatible OS.
Intel Macs running OS X also offer something called virtualization for running other OSs. This method allows Windows or another OS to boot up and run inside an OS X window, as if it were another OS X app. Due to modern CPUs from both Intel and AMD (Apple uses the ones from Intel), the virtualization runs pretty much at native speed, with no emulation of the processor needed, and the hardware handling requests from two operating systems simultaneously. For your SWMBO, this will likely be the only method needed, as the only advantage booting purely to windows offers is gaming or other graphics card dependent speed. Using virtualization will offer her the ability to run OS X and Windows apps side by side with some great integration. There are two programs that offer this and both are about $80. One is from VMWare, called
Fusion and a company called Parallels offers a product
after their own name. With either one, I'd recommend sticking with Windows XP. Running Vista side by side with OS X is not pretty, nor speedy.
There seem to be two schools of thought among the Apple cognoscenti. (1), that there aren't any Macintosh viruses (virii?) out there so no protection is necessary; and (2) that yes, there are some, and more will certainly arise so better to play safe.
I fall in a different camp. While some trojan programs and such exist, and some programs in OS X could be exploited to make viruses, it's not likely to ever be a big deal. OS X is built on Unix and inherits all the security setup from over 30 years of being used in multi user environments. I also don't buy into the whole school of thought saying Windows has virus problems because it has the most market share. Years back, servers were much better targets for viruses, since they were always on in an era of dial up. Microsoft never has had dominance in the web server market, but yet their product suffered way more virus issues then the leading Apache software. Hackers target the vulnerable systems, and Windows doesn't have a good track record of shipping tight systems. If someone could turn the Macs of the world into a spam sending farm using a virus or worm, they would have. Because even with the slimmer market share Macs have, it's still a ton of machines.
So, lets answer the question directly. As far as I'm concerned, none is needed on the OS X side. Use common sense, think about what is going on if the security prompt pops up (OS X has a worthwhile security setup by default, and more meaningful then UAC in Vista), and keep backups as normal. As for Windows, well, if you use the products above, and don't really have a heavy need for Windows, install the OS and needed apps, then back up the files that VMWare or Parallels make to a DVD or something. Do not do any web browsing at all inside Windows, nor e-mail. If a virus does get in, just wipe away Windows, and bring back the backup files. If you save all your docs from Windows on the Mac side, you can just drag them back. If paranoid about it, install
the free AVG inside Windows. Paying for virus protection on an OS not likely to be used heavily is just throwing money away when the free alternatives work fine.
Ok, with all that talk of running Windows, and virus protection, a more important question arrises. Does she really need to run Windows? Office exists for the Mac from Microsoft, and Apple's own iWork product also works with Microsoft file formats. E-mail, web and such all work fine on OS X. Why is Windows needed? Thats the first question that really needs to be answered before worrying about how to run and protect it.