If you don't want to change the password:

1 > Get a program called Ophcrack(very large. 496Mb)
2 >> Download the liveCD (ISO)
3 >> Burn the ISO to a CD using an ISO burner
4 >> The Live CD's come with the free rainbow tables, so you might need to download other tables (cost money)
4 >> Bootup from the CD
5 >> Crack the SAM and System file

Another easier way is to remove windows password.
Try Windows password Recovery Tool 3.0 , you can login back in 5 minutes with it!