Well, but many keys just contain information like install path etc. which tell the program that they are installed, nothing more. Warcraft III for example refuses to run when a certain key is not in the registry but runs fine if that key is added.
Anyway, I found a way to do it, I will post it here in case anyone else finds the need to do the same:
- Open registry editor (Start => Run => "regedit")
- Select HKEY_LOCAL_MACHINE
- File => Load Hive. Select the "software" file from the old WINDOWS\SYSTEM32\CONFIG folder (not the current installation, the one you backed up before)
- Enter a name you want (like "software.old", just not "software")
- Now you can remove any entries you don't want to transfer from the "software.old" key you just added
- Once you did so, select it and choose File => Export and save it as a file (e.g. software.old.reg)
- Open this file in an advanced text editor (notepad won't work, EditPlus or similar does - if you see many spaces between characters, it's not working) and search+replace "software.old" with "software"
- Use File => Import in the registry editor to import the file into your current registry