Try the stuff posted by hazel first, as long as you have or can get the windows CD's.
Reading your posts above you have a couple of possible options if you can get into that PC with a rescue CD.
The drive where the reg backup will be is the C: drive. The other one, "HP Backup" is a backup copy of the Operating System of that PC. It's for reinstalling windows if you screw things up. It's the same as having Windows CD's.
The only trouble about using it, is it puts the PC back to "Factory Condition". You would have to retrieve all personal data before running it. All music, photographs, personal documents etc.
Another option, Andavaris link, isn't as complicated as it looks. I repaired a neighbours PC using that method.
As long as the problem PC had System Restore running, it's a simple process of accessing "C:\System Volume Information", which is where System Restore files are kept, picking the last but one Restore point, and the navigate down to a folder called "Snapshot".
From that folder you copy the following files to another location:
_REGISTRY_USER_.DEFAULT
_REGISTRY_MACHINE_SECURITY
_REGISTRY_MACHINE_SOFTWARE
_REGISTRY_MACHINE_SYSTEM
_REGISTRY_MACHINE_SAM
Once copied to another location, you rename them as follows:
DEFAULT
SECURITY
SOFTWARE
SYSTEM
SAM
You would then navigate to "C:\Windows\System32\Config", and replace the similarly named files in there with these new ones.
These are the registry hives, and the only one probably corrupted is the "SYSTEM" file, which contains all the boot settings and configurations, but because the registry hives have a lot of interconnecting entries, you have to replace them all otherwise you could get other problems similar to the one you have.
If you have to go down this road, we can explain it step by step for you, but it really is pretty straightforward.
The pressing problem is getting into the computer.
If you can't get into that PC at all, there's another option to replace those files as long as you have access to another desktop PC.