I second that AMEN always back up you're files and data before any major
upgrades or upgrade to a new operating system
One more "amen". Before I moved up to W7 from WinXP,
I always backed up, if only to the on-disk Acronis
Secure Zone (a protected area on the main hard disk
where Acronis True Image allows backups other than
to exterior media), every time a batch of WinXP
updates came down the pipe. I did this because one
especially large batch of updates a year or so ago
put me back by a week or so when one of the updates
rendered my system unbootable (the eventual cure
was to do the big update batch in several segments,
but first I had to go to an external backup to get
a bootable system so I could retry the updates).
So even updates from the Mother Ship need to be done
with care and while wearing protection.