I'm not an expert on this subject but maybe you have programs running in the background? ComboFix is a major resource hog, check to make sure you do not have any programs in the background. Are you running ComboFix on your own without supervision from someone trained in using the tool? Have you made ay changes to your computer recently, such as installing new programs etc.? Try defragmenting your computer to see if that helps. I recommend MyDefrag to defragment your computer(Make sure you run this program without any other programs running in the background). Or run a windows defragment from the control panel.
From BleepingComputer.com (ComboFix download page):
"Please note that running this program without supervision can cause your computer to not operate correctly. Therefore only run this program at the request of an experienced helper."
Here's the webpage for MyDefrag:
I did some reading up and ComboFix can take around an hour to run. If there are no other abnormalities i would not worry to much. Once again I am not trained with ComboFix so head my words with a grain of salt (I may be wrong).
Edited by Alexander Caldwell, 11 January 2012 - 08:21 AM.