Well, I don't know what I did, but I am able to log in using that Administrator again without it locking up. I was hoping that deleting that account would help, but I ended up uninstalling a lot of programs and "utilities" that I thought would cause issues (TeraCopy, Eraser, TrueCrypt). I also removed an older Adaptec SCSI card I installed right around the time when the adminstrator log-in problems occured (again, it was only the admin account that locked up a few minutes after I logged in--all other accounts were fine), which gave strange errors during POST but seemed to work. I also uninstalled the nVidia control panel (or bits of it, as it doesn't seem to be a way to completely uninstall it) as well.
Thanks for the tips, though. I enjoy reading through the forum, and especially when it's not me having the problem.