I have had this trouble myself, and after working on it for over a week I thought I'd save TT members the same burden.
1. Go into Task Manager and list all processes in order of CPU usage.
2. ID the process that is sucking your CPU dry. If you don't recognise it, try running a tracing application to see what program / folder is running this process. I believe that WinInternals have an app for this.
3. Once recognised, try to end the process. If you can't run the KILL.EXE application from the NT resource kit (or on sites such as WinInternals) and kill the process.
4. Update your DAT files for your AntiVirus and boot into Safe Mode. Do a full scan on all local drives.
5. Boot back into normal mode and run a good spyware application such as AdAware. Its free, but ensure you have the latest DAT files for it.
6. Get the latest Service Pack and Hotfixes.
7. If you still have nothing, exam what applcation uses the process. Mine was DvISESL which is part of my communications software. Try to get the latest updates for the related software.
8. Maybe extream in some circumstances, but a uninstall/reinstall on the program did the trick. Just remember to backup the data!
9. If you are still stuck, try running a repair on the Windows installation. Many apps require Windows subsystems to run properly - if there is a corrupt file, then the repair maybe a solution.