I have solved my problem using the built-in admin account. For anyone else experiencing this problem, read my other forum thread here or simply follow this tutorial, and you should have your admin account fixed within a few hours.
A few days ago I started my computer and was taken directly to the Windows repair screen because my computer was apparently unable to start. After running the repair tool for a few minutes, Windows told me it had no solution, so I simply restarted the computer. After that it started fine and I thought everything was solved. But yesterday, after I logged in successfully, my mouse and keyboard suddenly stopped responding. I used the power button to manually restart my computer. I tried to log in again but I got the "User Profile Service failed" error message.
Now, every time I attempt to log in, I get the "User Profile Service failed" message immediately after entering my password. I logged in to the standard Guest profile and attempted to edit the registry according to the first method found here, but admin privileges are required to edit the registry, and every time I type my admin password into the administrator privileges prompt, nothing happens (it accepts the password but then does nothing). So it appears I cannot use my admin password to gain admin privileges.
Next, I booted in Safe Mode and attempted to log in to my admin account. It actually allowed me to log in using the admin password, but whenever I tried to click anything such as "Create an account", nothing happened. So it appears I can log in to the admin account with Safe Mode, but I can't do much else on that account.
I'm currently downloading Hiren's BootCD 15.2 because this discussion leads me to believe it might be able to copy files from my admin profile to a new profile without prompting me for admin privileges. Another problem is that I've gotten lazy so I haven't backed up files or programs in months, so I need to create a new admin account and somehow copy over all of my files and programs without accidentally losing them. I have a terabyte hard drive that's over half full of games and important documents, so I can NOT lose the data stored in my corrupted account.
Summary: I can't log in to my only admin account because it's corrupted. I can access it in Safe Mode but I can't do anything in Safe Mode (e.g. when I click "Create an account" nothing happens). I can't edit the registry from the Guest profile or do anything requiring admin privileges because the admin password doesn't seem to work. None of my files or programs are backed up. I need a method by which I can either repair my current admin account, or create a new admin account and copy over all of my files and programs.
Windows 7 Home Premium
avast! Free Antivirus (Is it likely the free version of avast! let a virus slip by and corrupt my account? I've never had any problems with avast! in the past.)
I've been running my home-built PC for almost two years now with no major problems. It's pretty quiet for the most part but I may have noticed some slightly louder whirring and clicking while booting up since these problems began. Then again, I could just be imagining it. I can post the exact hardware specifications if someone thinks the profile corruption could be caused by a hardware problem. When I have time I'll pop the case open and see if I notice any obvious internal damage.