Locked out of Admin account

My sister gave me her old Windows XP computer, but she deleted her user account before giving it to me. It was second hand when she got it, and whoever had it before her set up a password for the admin account. When I open the computer, it tells me to enter the password which I do not know. I've tried going on safe mode and then opening the run screen by entering windows+r but it doesn't open the screen. Advice?