I had a similar problem - 100% utilization leading to sloooow performance, tracked it down to a bad plug-in for IE7! I went into IE7 "Tools>Manage plug-ins", and tried disabling them one by one. Got it on 2nd try. The rogue plug-in also filled the temporary folder with empty (zero content) files, over 65,000 empty file names! Clearing those temporary files sped up the scanning by a considerable amount, and freed up space on the hard drive - an "empty" file is still allocated at least one sector on the disk.
Temporary files can be removed by opening "My Computer", right-clicking on the hard drive, select "Properties", then clicking on "disk cleanup". You can also look into the temporary folder and manually delete temporary files by using "My Computer" or Windows Explorer to go to:
C:\Documents and Settings\username\Local Settings\Temp
username is your User Name, and the "Local Settings" folder may be hidden - use the "Tools>Folder Options" menu to set the view to show or not show "hidden files and folders".
