System spawning dummy copies of apps with 100% cpu usage

I recently upgraded to a Core 2 6600 system, and I've started noticing some very odd behavior after my computer is running a few minutes.

Whenever I close ANY application (and when some are first executed), task manager spawns a second copy of the app's process, although the program seems to close correctly. The second process will use 50% CPU time (one entire core) until I end it through task manager. After that everything works fine, but this happens repeatedly. It seems to occur on and off, intermittently over time.

I've seen it happen with several different programs -- from task manager itself, to Firefox, to a couple of games, and even calc.exe (that little windows calculator app). If two of these "dummy" processes occur at once, it reports 100% CPU usage, although the computer doesn't seem to slow down much. The performance console is also showing 100% CPU usage, however, so I'm inclined to think it really is being used.

I am virtually certain that the computer has no viruses or malware on it, and the Windows installation is only a couple of weeks old. This just start about three days ago, and I can't think of anything I could installed that might have produced such a symptom.

I really hate to have to reformat as It took me the better part of a week to get the system set up the way I like it. Has anyone ever heard of anything like this before?

No, it's a standard 32-bit copy of XP Pro.