First of all, you need to BUY Windows98SE, unless you have access to a CD of
someone else's. Whenever this issue comes up in the newsgroup, people
recommend checking e-Bay for the software.
Backup: You need to back up all your data files. That means the stuff YOU
create, download or work with, like docs, spreadsheets, music, photos, etc.
The upgrade process won't force you to do this, although you may see a
dialog box with a suggestion.
In reality, you should be backing these things up daily, weekly or monthly,
depending on how sad or crazy you'd be if you lost it all. Your computer
will die at some point. It's not a question of IF, but rather WHEN. It