I have window xp desktop, I am not sure what changes I did on it... that when I restarted the computer, it gave a black screen with small window that says window log in. with user name HP_administrator and is asking for password. I don't remember any password. I tried logging through safe mode but it did not work and I tried pressing CTRL+ALT+DEL twice as well but no response. Can someone please help, I would appreciate. Thanks

This would be an indication that you have set a password on the CMOS. There are various ways to recover from this condition but it may involve opening the box.

My first suggestion would be to visit the HP web site or contact them to find out if they offer a utility to reset the password only. Many manufacturers do.

If that doesn’t work, the next thing to try is resetting the CMOS with the jumper on the motherboard. Some motherboards have jumpers for just the password and some only allow for a full reset. Check the HP web site for documentation about this. If you have to do a full reset, the CMOS will revert to it’s default settings and any custom changes that have been made will be lost.

If all else fails, you could try pulling the CMOS battery (if you can, some of them are soldered). Then cycle the power on the PC and reinstall the battery. This will also reset the CMOS.

When working on or near the motherboard, be sure the power is disconnected and that you have discharged any static electricity by touching something metal before touching the board.