How to remove Administrative Password without reformatting?

In my office one of user who use windows XP professional.
I configure him as a normal user, he always login by this user. I set administrative password that he never do any administrative work. Due to some software installation I gave him user as administrative right.in that times he may be change administrator password.
I don’t know this, so after I have removed his user from administrative group, I can’t do any administrative works.
He told me he did not change administrator password.
But it has been changed.
So how can I remove administrator password without reinstallation windows?
