beginners woes

My wife brought home a 4 year old Dell pentium 4 running windows xp pro. I hooked it up and it starts fine. But the only thing that comes on the screen is a sign in box for windows. We tried a few user names and passwords but they were all rejected. This pc came from where she works there were around 70 of them being exchanged for new pc's. They were used for inventory and such and most never were on the internet. Some of them were stored for a year or so so I doubt if we could find the person who used this particular pc. They did erase all the info they had on the pc's. But not being familar with xp (I have only used vista) I have no clue what to do. Is there a way I can go around this sign in box or maybe find the administrator and make a new sign in name and password? Thanks for any help. Steve aka skilit123