My computer frequently gets into a situation where a process locks up the
system by running away with the CPU.  The process usually shows up in Task
Manager running explorer.exe, though sometimes it's iexplore.exe or the login
program.

When this happens I can't kill the process with Task Manager or Process
Explorer.  It's also not possible to logout at this point.  The only solution
is to do a hard power down.

There doesn't seem to be any particular pattern to what triggers the runaway
process, though the situation seems to occur frequently during user login.  
Sometimes we never get past the screen where we click on the user name to
start the login.

Any ideas?