Which files do I want to backup from a Windows PC?

My mom’s Windows doesn’t want to boot anymore (even in safe mode) so I’m installing Ubuntu (at least for now). Anyway, obviously I don’t want to lose all her important stuff and preferably preserve the possibility to buy a new Windows and restore it in the previous state at much as possible.

However, I’m not a Windows guy, so my question is… Which files to I backup? I have an external harddrive. It concerns Windows XP.