Have you tried to look into the task manager just to see what process was causing that ? (can't be cudaminer if it is closed... could be windows explorer though... had something really strange yesterday while plugging my phone into my computer...)
Yeah, there isn't anything visible at all (this is *with* all processes shown for all users).
However I might be making some headway:
Previous settings:
GPU Clock Offset: 0
Memory Clock Offset: +500
I did this because I read somewhere that the GPU clock offset didn't seem to have much of an impact on hashing speed.
Anyway I just reset the memory clock offset from +500 to 0, and increased the GPU clock offset from 0 to 20. It was stable (but hashing slower). I cancelled Cudminer and it was responsive in closing. The system CPU usage wasn't through the roof after closing either. I repeated this with a GPU Clock Offset of 40. Same result. System fine. CPU usage looking healthy / normal.
I'm going to try to confirm this next, but this appears to suggest increasing the Memory Clock Offset puts a lot more pressure on the system both before and after(?!) mining.