In Windows Vista, SAM file is located in %SystemRoot%\System32\config\ folder. If the C:\Windows\System32\config\SAM file is deleted, the PC will stop halt and automatically reboot during system startup, right after user seeing the first sign of mice pointer. The Windows Vista PC will continue to reboot in endless infinite loop, causing system unusable. Even trying to get into Safe Mode will fail with automatic restart.
If the SAM file has been removed or deleted, either by intention or accident, Windows Vista has a backup copy of it that can be used to restore or recover the SAM file. It’s practically impossible to copy the SAM file from another system because it may contains the sensitive secret user password data. The backup or second copy of SAM file is located in C:\Windows\System32\config\RegBack folder.
To to get back the SAM file, simply copy the file from C:\Windows\System32\config\RegBack\SAM file. However, the default system SAM file has the size of 256KB, while the backup copy has only 64KB. Although file size is different, however it’s enough to allow system to boot into desktop without the need to reinstalling Windows Vista.