You are not entirely correct. There are no viruses for Linux, but there are quite a few exploits that other users or hackers can use to make your life miserable. Something like this is with Mac - the Mac OS is much more secure than Windows and bugs and exploits are fixed very fast and there are too few viruses for Mac OS so you don't have to worry about it.
About viruses on Windows. Even if you have infected files on your computer (downloaded from internet, from another Hard drive or something) i.e. - the "My documents" folder from the other HDD the virus itself have to be activated before it start making your life miserable. So if you have a PC with some decent and updated antivirus software (Avast and Avira are free and do good job. I don't recommend NOD32 or Symantec/Norton series) and attach an infected HDD you can scan it and get rid of the viruses in it. There are types of viruses that autorun when you attach a flash drive or an external HDD, but AV software can easily stop those. If you have your "My documents" folder on your mac and you don't need anything else, you can format the hard drive (whipe it clean) and after that you can copy back only the files you want. Pictures, Movies, Songs... viruses usually have some name weird names (like "827kdyvuwo19") or something to confuse you (like "microsoft updater") and make you think it is something important. They are usually .EXE, .COM, .BAT, .SCR files... (.INF for autorunners, but they are invisible) so you can easily choose which files you want to have back. Just to be sure - don't copy folders, but only files. Folders may contain virus files that are invisible. Still they have to be trigered and if you don't see them you can't run them, and since their triger is already gone (left on the infected drive, not on the new one) you don't have to worry.
What I do when install a new computer is - I split the hard drive in 2 or 3 parts and install Windows on one of them (usually the one with lowest size). I instal programs in Program Files (C:\), Games on D:\ and anything else (pictures, working files, audio/video files...) on E:\. If my PC get infected and I can't clean it - I just format C:\ and instal a fresh Windows copy. D:\ and E:\ stays intact and I haven't lost anything. If I get some very nasty virus it may affect all .EXE files and I have to wipe out D:\ also, but I still can install my games again and play them, so it's not a big deal.
Hope this helps
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