Boot with a System Repair disc, select the correct installation, start a command prompt and replace the
system32(maybe you should create a backup first):
The drive letter shifts up by one because C: is occupied by a system reserved partition that is normally hidden. Then boot the computer with your Windows 7 installation and click on the small pie-like icon in the lower left corner of the screen. This now starts a command prompt instead of
copy d:\windows\system32\cmd.exe d:\windows\system32\utilman.exe
net user, you can list all users and with
net user MYUSERNAME MYNEWPASSWORTyou can set the password of any user.