It's not an "hijack". If the installation of the SB software had gone correctly, you wouldn't be seeing any values in "RunOnce"--the key is used to run the final stages of an installation following a reboot and the setup program is expected to delete the values as well as temporary installation files.
Since you say it works with the drivers, uninstall the Creative software and (only if necessary) reinstall the drivers.
I'm at home on my Win98 machine so I can't speak to XP, but with 98, you enable/disable the Windows volume icon on the first page of the Control Panel Multimedia applet.
O4 - HKCU\..\RunOnce: [DefaultP17MIDI] MidiDef.Exe
O4 - HKCU\..\RunOnce: [DefaultP17] P17Def.Exe
I think these are associated with Sound Blaster. Do I need them?
By the way I have the weirdest problem with SB. After installing it and rebooting as instructed, I get to the XP welcome screen. All is well until I click my user name and the XP welcome jingle comes on. It gets through the beginnning but then suddenly shuts off. I then notice that the sound doesn't work anymore. What am I doing wrong? I installed the drivers and the software but it doesn't work.
But I have figured out that if I install the drivers only, the sound works. Once I attempt to install the Creative software it doesn't work. All I wanted was the little "C" volume icon thing instead of the default grey speaker.