Can I "undo" my Vista install?

I seem to remember that when I upgraded to XP there was an option to revert
back to my previous OS if I messed up.

Also, my prefious OS installations always preservid my data so I guess I've
gotten a little complacent. I backed up some really important stuff but
there's lots that I still need, and I can't get to it. It's probably in the
administrator account that I'm locked out of. I was administrator in my XP
installation and without password access, but now I have a blank slate in my
named user account and Vista will not allow me to access the admin account.

Am I simply SOL?