Windows XP Home password removal

Hello, and thanks for your help in advance. I recently purchased two ex demo laptops from a computer retailer that was closing down. I did not get a chance to look at them until today and have found that they both have passwords on them. The retailer has ceased trading and I have no contact details for him. One is a WinXP home, the other is a pro version. I tried logging in as administrator and guest by booting into safe mode, but it seems these accounts have been removed or locked down.
Can anyone suggest a way that I can bypass the password authentication so that I can log onto these computers. Thank you.
Is my only option reformatting? Thanks.