Need Direction please help

I am completely new to this and I have an idea of a utility i am looking to create.

I dont think it would be that hard, but since I am new it is eluding me.

All that I need is a small util that while logged into the Local Administrator account, running the .exe will change the password from whatever it is, to a "set" password. I dont want it to use any domains or OU's just that 1 local admin account.

Id like it to work on Vista, and XP. Ive tried the Net User command and i keep getting access denied.

Any help would be greatly appreciated.