First you should to boot with your CD. Click in Repair your computer. Press shift + F10. Then backup the sethc.c, the PATH is c:\windows\system32\sethc.exe. Now you can replace the cmd.exe in the SYSTEM32:
copy c:\windows\system32\cmd.exe c:\windows\system32\sethc.exe
Reboot the system and press shift five times. This start a command prompt. So you can use the following commands for reset the administrator password:
net user Administrator *
