Hey, pretty much the same thing here. I uninstalled a program that wasn't even being used and my CPU usage went back to normal.
I just recently had this issue with my CPU sticking at 100%. AVG was the only thing eating up my processor, but it wasn't any different than it was when I first installed it a few months ago. I uninstalled one of my programs (hotspot shield) that wasn't even running (just figured it was a waste on my hard drive as I wasn't even using it) and as soon as it uninstalled...CPU usage drop WAY down...back to normal.
So it turned out that a program that wasn't even running (even in task manager it wasn't using cpu) was causing my CPU to spike...I still don't understand it. A few days ago crash doctor told me the program had crashed and couldn't fix it and after I got rid of crash doctor (it wouldn't go away, it would keep popping up on the screen when I closed it, constantly telling me the program crashed and couldn't be recovered) the CPU started spiking.
So I guess my advice is when your CPU starts spiking and sticking at 100%, check all your programs and make sure they're running right. I'm not even sure what happened to Hotspot Shield, but that one program I didn't even use (even removed it from the startup folder) was causing my system to stick at 100%.