This instructable will tell you how to change an administrators password, even if you have a limited account. Probably this has been done hundreds of times on this site, so bear with me.
Step 1: Instructions
Open Command prompt/MS-DOS, (it's in accessories or you can go to run, type cmd)
net user "Any Username" * Note: Make sure to spell exactly what the username is.....nah
Type the new password, the text you type is not going to come up on the screen but its there.
It will ask you to confirm....do so