This will run in the background if you have the "Display options" icon on the taskbar. You can uncheck this by going into display settings, clicking Advanced, and unchecking the box that says, "Show settings icon on taskbar". when you reboot, rundll32 wont be in there anymore.

This is the monitor icon that you can right click on for quick acces for chaing screen resolution and color settings.

It may ALSO be from one of your other background tasks. if that doesn't take care of it, then run msconfig, uncheck things one by one till it goes away, and you'll know wich one it is.