My Win 7 pc boots fine in normal mode, but in safe mode, once it gets to the blue "Loading user account" screen, it errors out with a "User Profile Service service failed the logon" message box and after about 2 seconds reboots by itself. What might be the reason
It sounds like your computer can't login to your network with your user profile, but more info is needed to diagnose. The following forum discusses the error and provides a detailed solution:
If you can boot in normal mode then try to active the
You can try to fix with Safe Mode - repeatedly tap F8 as you bootup. The ADMIN account in Safe Mode has no password by default (so unless someone changed the password it should be available).
Some programs like the Google Updater (if you added the Google Toolbar, Chrome or Google Earth)
has been known to cause this issue.
You can find more information on Microsoft windows 7 security forum.