Almost none of the HP and Compaq consumer desktop computers do allow power-on passwords. Instead, they use a BIOS Lock Supervisor password and Windows Logon password. Follow Justin's advise for the Windows password, and if nothing else try to clear the CMOS password. For that, follow the steps on here. Hope this helps, good luck.
You should be able to boot into safe mode and log into the Administrator Account and do a system restore. I am going to assume you are using Windows XP.
1. When you power up the computer start and keeping pressing F8 every second until you get to a menu that has boot options.
2. Choose Safe Mode
3. When it boots up choose the Administrator Account
4. When this opens up it will ask you either to continue to work in safe mode or restore your computer to a earlier time using system restore.
5. Choose the Yes or No that takes you to System Restore.
6. Follow the prompts and choose to restore your computer back to a date before you were messing around. I always do 2 weeks just to be safe. None of your files will be affected however if you have installed any new programs or updates they will need to be reinstalled.
Hope this helps!!!!