The straightforward way to run something from the registry is under the Run or Runonce key.
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\Curr entVersion\Run
I believe msconfig displays the entries there. The advantage is that you can temporarily disable an entry, and then re-enable. In Windows, there is the Startup folder. Anything in it will start every time Windows starts.
You can search on "C

ATI\Support" in the registry.
Other startup files Windows uses are WIN.INI and SYSTEM.INI. Maybe WINSTART.BAT. You can do a search of text within all files.
Good luck. For me, every time Windows starts, two windows of the "My Documents" folder open, and I have found no reason why so far. If I don't close them, I get 4 the next time. Etc.
Is there some particular reason you need Explorer to open full screen? Or do you mean the desktop doesn't open full screen?