The only value from the registry I could help you out with specifically is
Start>Run . Regedit
HKEY_CURRENT_USER > Software > Microsoft > Windows > CurrentVersion > Run and Runonce
HKEY_LOCAL_MACHINE > Sofware > Microsoft > Windows > CurrentVersion > Run and Runonce
You can clear out anything from here that you dont recognize, and you wont screw up your OS (look closely at the names though, sometimes, they are odd, but recognizable)
As for Half Life 2, I dont know the specific location, because
a. I'm at work
b. I don't have HL2
I don't know if that will help your situation, but it is useful knowlege nonetheless.