your Mac partition will be untouched. They have nothing to do with one another aside from sharing space on a hard disk. The Mac does not use Windows resources and Windows can't read Mac drives.
Your likelyhood of getting a virus, malware, etc. while running Windows on your Mac is dependent on how you use it. If you get lots of spam you are at risk. If you automatically open emails rather than view only the subjects you are at risk. If you download torrents regularly you are at risk. If you surf to questionable web sites (hey, you know what they are), you are at risk. These situations are launch pads for malware. When you don't do unsafe computing you are a lot less likely to get a virus or other bug. That said, hey, if you are gonna do unproductive things on your Mac, at least do 'em in OS-X. Use Windows for the tasks you cannot do on your Mac and you'll be a lot safer.
As far as the difference between Boot Camp and Parallels et al, Boot Camp doesn't allow you to access your Mac files and programs while running Windows and the VR programs do, but at the expense of speed. Both make your Mac appear to be a PC from the ground up, drivers, peripherals, control key, everything. Obviously Boot Camp costs nothing, but both need a licensed copy of XP or Vista to work.
I'm using Boot Camp, but I do wish I could access my Mac data sometimes, like when I email myself a password and it's stored in Mail...can't look it up in Boot Camp Windows though you can with Parallels. But then again, Halo2 plays better during a Boot Camp session than running under Windows via Parallels.