XP->Vista Upgrade, now can't use old accounts

I had four accounts on my XP Professional system. After upgrading, I cannot
access any of the four *unless* I log into Safe Mode. From Safe Mode, all
four function normally. Without Safe Mode, all four immediately shift from
"loading your personal settings" to "logging out"--and right back to the
choose-your-login-id panel.

The Administrator account, which I finally (after four hours) was able to
make visible on the login page, can be used to login even outside of Safe
Mode. I have not seen any problems with devices, display modes, or other
clearly visible components.

How can I diagnose the login problem further, in order to restore use of my
old accounts?