xp pro on optiplex gx110

Hi

I don't even know if i am putting this in the right area. I  just recently purchased the computer i am inquiring about. I am not on it yet. It has windows xp pro on it and i can't get past the Log on Window cause i don't have the  password, but it does show the user name, and the firm that the system was used in. It was from a law firm, so i have to do some redoing once i get past the password part. Take it out of networking and just put it for home use only.

How do i get past the password part.

What do i do to solve this.  Do i have to reinstall that program or no. It does have a service tag which shows it in your system, has been modified like i said to windows xp pro and also has more ram then the original and a larger hard drive setup.

Any assistance would be greatly appreciated..

Thanks

Carol