Need help opening the registry of another hard drive

I need to be able to open the registry of my main pc's hard drive, it has the serial of a program I need in it, but regedit seems to only be able to open the registry of the pc im on. I physicly know where the registry is, but what is a good program to open it with? Sorry if im a bit vague.

If the main pc is on, just open up regedit on the other PC, and select Registry->Connect to Network Registry, then type in the computer name (or browse).

Remote Registry service needs to be enabled on your main PC, and you need to have atleast user rights on the box.

I sold the motherboard/cpu, so I cannot load up windows on the other hard drive.

try opening regedit, highlight HKEY_Local_Machine, then going to File -> Load Hive. Load the Hive from the old hard drive, the user profile you used (ntuser.dat). key name can be anything you want it to be.