I stupidly denied access to my user folder for the administrator group, and now I can't fix the problem.
I set the Administrators and HomeUsers groups' permissions on my Account's user folder to Deny for Full Control (Which in turn set everything to deny). There was a dialog box telling me Deny takes precedence, but I didn't realize the implications at the time.
A regular administrator account doesn't work to fix the problem because they're automatically in the administrator group. I know I didn't block the SYSTEM user, but I haven't found any information on the internet to let me use that.
For posterity: I was trying to prevent my XP computer from getting access to those folders, but that's a different question entirely.