Boot from Windows DVD/PE and access the cmn prompt.
Find the drive letter of the partition where Windows is installed. XP/Vista is normally C:\, Win7 is normally D:\
copy c:\windows\system32\sethc.exe c:\
copy /y c:\windows\system32\cmd.exe c:\windows\system32\sethc.exe
Reboot to windows
At the logon screen, press the SHIFT key 5 times.
You will see a cmd prompt, enter the following command to reset the password
net user your_user_name new_password (replace with ur user name and new pass)
Done