I found it by accident, and I don't know if it was mentioned before, but if you change the sound preferences of your VM file, and choose "null" for the input sound device, then Parallels stays on 9% of proccessor usage when idle, and on 2% when paused, otherwise it's on 18% idle and 10% paused, at least on my Core2Duo MBP.
Thanks a lot, that worked! Windows XP under paralles now uses only 8-9% of CPU (out of 200%) when idle and 2-3% when paused (as oposed to resp. 15-20% and 5-6% with sound input enabled). Quite an improvement. Also, don't forget to disable any USB device you don't need in Parallels as they cause massive CPU usage for nothing. I just bought a USB printer and parallels connected to it automatically when it launched causing it to use 35% of CPU idle (even though i haven't even installed the driver for the printer under XP). Simply going to Device -> USB and selecting Deskjet to disable the printer caused the CPU usage to be divided by 4...
It's not in Windows but in Parallels menu bar at the top of the screen. And it's actually not Device but Devices (sorry for the spelling mistake).
You have to STOP the Guest OS. From the Host OS within Parallels click on Edit and then Virtual Machine. You'll see the section for Sound Device. Select Null for Input Sound Device from the drop down list. Hope this help clarify things for everyone.
