Not sure what you mean by startup, is this when the PC switches on, or once you get to the XP\2K logon screen.
If its the former then its a case of open the PC and look inside for a small flat battery, remove it and flick the power siwtch a few times, then replace it and power on the PC, this should clear any passwords on the BIOS, also there may be a jumper on the board that will do the same job, I suggest either reading the manual for the motherboard or if you dont have it, google the board and download it, doing this will reset the BIOS to defaults so you will need to reset the time\date and a few other things.
If its the OS then there are several methods to removing the password, one is a bootdisk from here, its a bit complicated to do and you must read the instructions, but it does work.
Another method is to delete the sam file, this stores all passwords on the system and is found in the path c:\windows(winnt)system32\config, however this method requires you to slave the drive to another Install, as the file cant be deleted when windows is running.
Lastly there are brute force programs that will decrypt the password, but are complicated to use and require a knowledge of the OS.