I dont think you can customize MSCONFIG per user. What you can try is create a different user account for game-playing, then control user specific startup items. See this tutorial (Method 2, option 1).
Startup Programs - Change
In addition, keys of interest in the registry, again user specific will be the following:
HKCU\Software\Microsoft\Windows\CurrentVersion\Run
HKCU\Software\Microsoft\Windows\CurrentVersion\RunOnce
HKCU\Software\Microsoft\Windows\CurrentVersion\Policies\Explorer\Run
(HKCU= HKEY_CURRENT_USER)