If you install a driver, program or just accidentally delete an entry in your registry, Windows can blue screen on your next boot and won’t properly recover. The simple manual fix here requires you drop the hd into another machine or have another bootable disk in the original computer.
1) Pull the busted boot drive out of the bad system and put it in another machine that is working and bootable or just switch bootable drives if you have two boot drives in your pc.
2) Set the permissions in the System Volume Information folder on the boot drive you are fixing to readable by everyone (you should delete the everyone entry when done). If you have xp home, you may need to download this file ( SCESP4I.EXE ) and once extracted right click on the setup.inf to install it -> http://support.microsoft.com/kb/195227/EN-US/ . This will give you the security tab in Windows XP Home that is otherwise not available. Note, you must also be able to see hidden system files which you can change under tools -> folder options -> view -> show hidden files and folders and hide protected operating system files.
3) Once you can get into the system volume information folder, go to the restore folders. Each folder is an RP folder named for the restore point instance. If you look in a detailed view, just select the last RP point by time before your computer went to hell. Go into the RP folder and the snapshot folder.
4) Now you are going to copy:
_REGISTRY_MACHINE_SAM -> %SystemRoot%system32configSAM
_REGISTRY_MACHINE_SECURITY -> %SystemRoot%system32configSECURITY
_REGISTRY_MACHINE_SOFTWARE -> %SystemRoot%system32configsoftware
_REGISTRY_MACHINE_SYSTEM -> %SystemRoot%system32configsystem
_REGISTRY_MACHINE_.DEFAULT -> %SystemRoot%system32configdefault
5) Put the boot drive back in the PC and/or switch bootable drives. Reboot your machine and your computer should now have a recent restored registry.