On one of my XP machines, I've been having 100% CPU busy hangs when the machine is left overnight (and occasionally while running). When they occur, <Cntrl><Alt><Del> does bring up Task Manager, and if I end the task that's hogging resources, I'm OK until I click on something else, at which point THAT task becomes 100% busy.
This sure sounds like it's spinning on a lock.
Looking at Application level events in Event Viewer, I find that nVidia nView is having trouble with timeouts on mutexes, and these always start with mIRC. So for the last few nights, I've closed mIRC and all has been well.
That's anecdotal, but interesting. This is mIRC 6.16; I just installed 6.17, will see if it helps, but I'm wondering if anybody else has seen these same problems and suspected mIRC.