The Windows passwords are stored and crypted in the SAM file (c:\windows\system32\config\). In the same folder you can find the key to decrypt it: the file SYSTEM. This two files are locked by the kernel when the operating system is up, so to backup it and decrypt you have to use some bootable linux distro, to mount the disk when the system is down or to use some program like fgdump, pwdump or meterpreter hashdump. Someone told me even that is possible to copy this files causing a Blue Sceen of Death an then remotely dump files, but I never try it.
An alternative, when the operating system is working, is to take the two twins files present in folder c:\windows\repair\ that the system create as a backup. This work up to Windows XP (think also Vista), but I can’t be able to find these files on Windows7. If you know something more, write me.
Once you have the files you use bkhive to extract the bootkey:
Then put together the bootkey and the SAM file:
And then try to crack the hash:
This is just an exemple of use of this tools. To crack hash you can algo use google that is always the bigest resource.