Download the free version of Ad-aware 6 at Lavasoft.com. It will be able to remove those pesky spyware files that Windows is using, and this includes registry changes. Remember to update it with the built-in updater before you let it scan your system. Whatever it finds, and it may find alot, is not needed for Windows to operate, so don't be afeared to let Ad-aware remove it. Besides, it makes a backup of whatever is removed should you ever want to restore something. Some programs though, need their spyware to function, like Kazaa. These kinds of programs integrate well with my recycle bin.

Ad-aware will prompt you to let it run after a reboot so it can remove the remaining files that were being used by Windows - it will run before anything else loads.
Unless your son is still running/playing Diablo on that machine, you should be able to safely delete those Diablo files.