I recently obtained a used windows 7 computer. The only visible user had administrator privileges, and its name was that of the previous owner's. I changed the name via User Accounts ► Change your account name, but for some reason that revoked my admin privs. Now, whenever I try to do something that requires admin privs a box pops up that wants me to enter an administrator password, but doesn't have any place to type any password.
After a bit of googling, I found a few places that said the first step to fixing this was to boot into safe mode. I restarted the computer and pressed F8, but that didn't do anything. The only other way I saw to boot into safe mode uses msconfig, which needs admin privs. I tried changing the name back, in case that helped, but that, too, requires admin privs.
How can I get my Administrator privileges back?