The default password for Administrator is no password (blank). Try that.
It sounds like when you got it, they had set up some user account that does not have Administrator privileges and that is what you are using to login? A limited use account will not be able to do a lot of things and (if you are like me) you will get frustrated fast.
If you can't get logged in as Administrator to change your account or create a new one, I hope someone at the church knows the Administrator password. If they changed it and don't know what it is, that can be a headache sometimes. If it is your computer, there is no reason for them not to tell you what it is - if they even know.
Once you get it sorted out, you should change your login account to be like an Administrator account or create a new one and leave the "real" Administrator account alone and use it for emergencies - not just login as Administrator all the time. Since you now own it, consider changing the Administrator password back to none because that is what people that try to help you are going to hope it is - empty.
If you assign or keep a password to the Administrator account, you better take some measures not to forget it, or you will be back here later with a "I changed my Administrator password and can't remember what it is" subject line for a future post.
Since it should be an account you rarely use, remembering it in your head may not be sufficient (hint).
While you are talking to them, see if they have the original XP installation CD - this may come in handy for later and you can't just "make" one from what you have. Get all the CDs they might have that should go with the computer, but the XP installation CD is important if your ever can't boot on your hard disk. I would really hound them for this if they don't seem to interested in producing it.