Ugghhh. I just upgraded my computer and received the error, "The User Profile Service failed the logon". This disallowed me from logging in. After the panic settled, I grabbed my Kindle tablet and Bing'd the error message. Fortunately, it didn't take long to find the steps to fix this on http://support.microsoft.com/kb/947215.
It did involve rebooting into Safe Mode, digging into the Windows registry, modifying some settings, and rebooting. Thankfully, there was no more drama.