I think anyone who steals computers would know this anyway, so I'll risk it by telling you how.
Turn off the comp AND remove the power lead. Now open up the case, touch a metal part to prevent static damage, and take out the motherboard battery - a small silver disk about the size of a 10p. Note which way up it is.
Leave for 30 minutes, then refit the battery and connect the power lead. Start the PC and enter set up. Now you can enter your own password (1234 is good!) and off you go.