I have this problem too and have done the following to pinpoint it as being with IE 8.
Opening Task manager before starting browser then sorting by CPU and monitoring CFPU usage whilst browser is in use and after closing.
I have found the following:
While browsing normally there is no problem. Typically IE and Firefox both briefly go up to 50% cpu whilst starting and occasionally whilst opening a page but only very briefly.
However after closing IE or after closing a single IE tab an instance of IE on task master flits about between 40-50% CPU. If I close multiple tabs simultaneously i get an instance of high CPU for each closed tab. this immediately causes lockup of laptop only fixable by reboot.
Note this is occurring after IE is no longer a running application. The instance of IE on high CPU usage persists indefinitely long after browsing has terminated. Even when nothing is showing on the Applications tab as running at all.
I have found the only way i can use IE is to use task manager to close this high CPU after each tab is closed and to only use a single tab at a time.
I have further experimented by individually closing browser addons and found that the problem also happens when none of these addons are enabled. So its not an addon issue.
I have moved up to the latest IE 9 beta and the problem continues. Other issues with that browser led me to revert to IE8.
The problem began around about the turn of the year and I suspect some AutoUpdate to have been responsible.
Anyway i now only use IE for some online banking applicatiosn I use requiring Activex controls.
I use Windows 7 Home 64 bit, MSSE 2, regularly run Malwarebytes and Superantispyware so do not belive its down to any malware.