Boot with a System Repair disc, select the correct installation, start a command prompt and replace the `utilman.exe` executable with `cmd.exe` in `system32` (maybe you should create a backup first):
```bash
copy d:\windows\system32\cmd.exe d:\windows\system32\utilman.exe
```
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 `utilman.exe`. With `net user`, you can list all users and with `net user MYUSERNAME MYNEWPASSWORT` you can set the password of any user.