I'm not sure this is a correct way or not, but put a battery out from mainboard and wait for a min before put if back and open your PC again should remove password.(I'm not sure this will help or not.) :)

http://home.eunet.no/~pnordahl/ntpasswd/

This. Download and burn to a disk, boot from it and follow the instructions to reset account passwords. Exploits the fact that Linux-based OSs don't respect Windows protections.

If it doesn't work, reformatting is the only real option. If you can't get at your data in Windows to save it before reformatting I suggest using a Linux LiveCD.

I don't think that will work, it would if it were a password on the BIOS but it's something with Windows.

Never been in that situation, you really shouldn't forget/never know the admin password unless it's some work computer the IT department is taking care of.
Try googling for things along the lines of reser / recover forgotten windows password. I'm pretty sure there are 3rd party tools to get around it as long as you have access to the computer.
If you have the windows setup disk, you can try the 3rd method listed here: http://www.askvg.com/how-to-reset-recover-forgotten-windows-nt-2000-xp-2003-administrator-password/

don't think you can do a system restore unless you can get in as the main admin..

I don't know if this link will help but good luck..

http://4sysops.com/archives/how-to-recover-a-non-bootable-windows-installation-the-microsoft-diagnostics-and-recovery-toolset/

A m8 of mine bought a laptop from an old friend. the admin password had been long forgotten so I couldn't help him set it up properly as a guest.

After a good while of trying to get in as admin it seemed the only option was to re-install the OS. This was fine because being recently bought second hand there was no information he wanted to keep.

If you got files you want to keep, throw them onto an external HDD and re-install the OS.

You like to have everything under your control I see! =P