When HOSTS files go bad
For months now, myself and a colleague have had frequent lockups on Windows 2000 about every hour or so. It's almost always the same time past the hour, and all open applications stop responding - you can still move the mouse, but everything else just flashes in the taskbar or starts the system beep off when you click the mouse buttons or attempt to type.
The only clue we got is that when it did happen, the CPU utilization would swing up to 100%, most of which taken up by SERVICES.EXE. The question was, what service was causing the disruption?
To cut a long story short, it turned out to be our HOSTS file causing the problem. Tucked away in C:\WINNT\system32\drivers\etc it had been expanded hugely (to 300kb) by Spybot adding dozens of entries for dodgy sites; getting them to point to 127.0.0.1. My guess every so often the OS checks the file for updates and spends a while adding the entries to some internal table - which causes the freeze-up.