Never use Windows, Mac, or iTunes. But, what I used to do, for a friend, was to locate the 'folder' where all the tunes have been turned from mp3's, to iPod format tunes.
Once you have that folder, you can copy all the tunes to an external USB Stick. They are cheaper than a external hard drive, and come in 16GB/32GB Varieties. That might not be enough, depending on exactly how many files you have in the folder.
As for using different accounts. That is a good idea. Because, last time I looked, iTunes limited the amount of different PC's you can use the same account on.
Do a google search for 'iTunes music format extension' and see what you need to search the folders for...it would be a search string similar to *.mp3 (use the asterisk, that is the wild card search pattern) but it will not be .mp3, it will be something else. That is why you have to search for the iTunes music format extension. Then, when you have that, search for *.xxx replacing the .xxx with the extension for the iTunes music format.
Jesus, that was long!