I've never had this happen because of a software install, I'd reccomend:

1. Hit F8 as if you were going to safemode, and select "last known good configuration"

2. if you use the default XP log-in style (I forget what it's called but it lists your accoutns and a photo) hit CTRL+ALT+DEL twice to bring up the bring up the traditional style log-on, that might help and it will also give you the opportunity to use the "Administrator" account if the problem is with your log-on.

3. If neither of those options work your best bet is probably an XP "repair install" from your disks.