I really hate to say this but I must.
This is taken from the following HP link.
A Password is Required Causing the Computer to not Startup or Boot in Windows XP - HP Customer Care (United States - English)
Power On
The Power On password is set in the System BIOS options. It prevents unwanted users from accessing the computer. By default, HP does not set the Power On password. If you set the Power On password in the System BIOS options you will be prompted for it immediately after you power on the Notebook PC.
If this password is forgotten, a repair at an HP certified repair center is required to reset the computer. Please contact HP to schedule a repair. This service is not covered by the warranty
So the problem I have is that the guy you bought it from does not know the password and HP says they don't set one by default. This implies that the possibility exists that the guy you bought it from may not have owned it as you say "Neither myself or the man I bought it off know the password".
If it's a Windows logon password and not a BIOS password then it's still the same situation. The seller would or should know it.
A comp that's playing up is suspect too in light of everything else.
As much as I hate to say it this might be a case of the seller not having rights to this computer. That's why we have certain rules pertaining to password issues.
Please read Rle 5 of The Rules to see why.