I manage my neighbour's iPod from my computer and safely store my own MP3 files on the same machine.
Seems unlikely that installing iTunes would wipe your tunes unless they were in a folder with an identical name to that being created by the iTunes installation and it shouldn't wipe that without asking (but with Apple's weird attitude to filing systems, who knows?).
iTunes does do some devious stuff - like any files copied to the iPod will also be buried below several layers of directories on the computer. Can be found by Windows Search if you know a filename -- or by looking for *.MP3 and then creating a shortcut to that directory/folder on the desktop.
The other criminal negligence on iTunes part is that it's all too easy to wipe tunes
from the iPod if they've been deleted on the computer -- I don't know if that works the other way too.