Win XP reinstall aborts (w/CD)

I'm trying to reinstall XP by booting from the CD. The problem is that when I
type in the admin password (which I'm sure is correct), the computer goes to
c:\windows, aborting the installation.

I'm reinstalling because a bunch of things don't work after I used a
registry editor (I know, bad idea) including updates (which hang), help and
search (which don't work at all) and system restore. SP1 and SP2 were
installed successfully before the problem started.

I tried reinstalling from the CD while in Windows, but get stopped because I
have a newer version installed (I assume it's SP1/2).

Any suggestions? Thanks.
Beth