Without an administrator-level account you can log onto, you can't install new software, edit the registry, or really maintain the PC in decent condition. Windows won't just tell you someone else's password, or let you change it to something you know, but there are ways.
Here's a simple one:
You'll need the Ultimate Boot CD for Windows. Downloading and preparing this great tool is a bit complex (see Six Downloadable Boot Discs That Could Save Your PC for more information), but it's well worth the effort. This is really one of the most useful tools around.
The program you want is Vadim Druzhin's NTPWEdit. Once you've booted the Ultimate, select Start, Programs, Password Tools, NTPWEdit. Vadim To change any account's password, simply click (Re)open, select an account, click Change Password, enter the password of your choice twice, click OK, Save changes, and Exit. Then reboot and enter the password.
Does the existence of such free software make you nervous? Relax. The program can't tell anyone what your current password is; just change it. No one is going to steal your account without you knowing it's been done.