Thought I would spend some time on this. Searched internet for the problem and found that it is not an unusual problem. There is no consistant fix.

Here are a few things that seemed to work:

Open Task Manager to see if you can identify the process causing the problem. If so kill the process to see what happens.

There was one instance where an old PCTEL modem in a Dell was the problem. They couldn't find a driver that would work, so changed out the modem for a hardware modem and solved the problem.

One was strange so I will provide the information as posted: After trying various solutions,
i suceeded by changing the following registry entry:
hkey_local_machine\software\microsoft\windows current version\run
change pctvoice to be
xpctspk.exe
June 15, 2002 03:14 AM GMT [ unassigned ]
i was having a similar problem (explorer suddenly useing 99% cpu), i am connected to a network of over 100 pc's, i had a folder (named 'shared' but not actually shared) that contained 3 shared folders and heaps of sub-folders. these 3 folders were originally on my desktop but i put them in the 'shared' folder to neaten things up a bit, thats when the problem started, if i so much as touched the 'shared' folder or any sub-folders, or any files in them, then the cpu would shoot up to 99% and stay there until a reboot. It wasnt a virus (i ran a scan), or anyone accesing my hdd (happened when network cable was unplugged). It took me a week before i tried putting the 3 shared folders back on the desktop, and deleting the 'shared' folder, and now the problem is gone, i havent had any worries for the last day, so i assume its gone.
hope this helps a little.

Also know that more than one incident involved Explorer.exe. This is one of the more promising posts:
If Internet Explorer (IE) 6 is slowing to a crawl and/or hanging and starts to use 100 percent of CPU time it may well be a corruption of the "Temporary Internet Files folder". In particular, trying to delete Temporary Internet files via Tools, Internet Options or other privacy software seems to trigger this behavior. Also, attempts to view files in the Temporary Internet Files folder may show the folder as already empty when it is not. This behavior occurs because the Temporary Internet files database is corrupt.

The Temporary Internet files aren't really files but entries in %systemdrive%\Documents and Settings\%username%\Local Settings\Temporary Internet Files\Content.IE5\index.dat. Deleting that file solves the problem. This same problem has occured for NT users and the fix works for XP.

1. Open a command prompt window on the desktop (Start/Run/command).

2. Exit IE and Windows Explorer (iexplore.exe and explorer.exe, respectively, in Task Manager, i.e - Ctrl-Alt-Del/Task Manager/Processes/End Process for each).

3. Use the following command exactly from your command prompt window to delete the corrupt file:

C:\>del "%systemdrive%\Documents and Settings\%username%\Local
Settings\Temporary Internet Files\Content.IE5\index.dat"

4. Restart Windows Explorer with Task Manager (Ctrl-Alt-Del/Task Manager/Applications/New Task/Browse/C:\Windows\explorer.exe[or your path]) or Shutdown/Restart the computer from Task Manager.

Good Luck big guy.