Is it the password set by the BIOS? or just the Windows user account password?

Ahh okay.

The password can be removed easily, but it will involve opening up the machine casing and using the hardware.

Power down the machine, and remove the casing. On the motherboard, there's a small round battery, remove that for roughly 10 seconds and put it back. That will reset all the CMOS settings including any passwords.

It will look something like this, so it's hard to miss.

Hmm, how old is this PC? it's a desktop PC right? if not a battery, then something else will be there in it's place, maybe a small jumper, see here:
http://www.computerhope.com/issues/ch000235.htm