loginwindow process out of control
Alright these are the olden goldies for fixing stuff like that:

1) Repair Permissions:

This is often a bit of a sacrificial chicken but in this case it could very well be that something has caused the permissions on the login database or an associated file to end up corrupted.

2) Verify the disk:
It's unlikely to be a disk problem but whilst you're in there you might as well clear that off.

If they don't do the trick once you're logged back in go to Console in Utilities and post the log files up here, there might be some useful information in them.