My girlfriends computer (made up of lots of bits of my old computers) has been acting weird for some time in a way that defied logic. Swapping around the memory would cause it to crash randomly, but the memory was okay. Sometimes the machine would fail to turn itself off. Sometimes the machine wouldn't turn itself on. Sometimes it hung for no reason at all. After swapping various hardware, changing BIOS settings, and playing around for months on and off with no effect I finally gave in and bought her a new PC from DELL. We plugged it in last night and it showed almost identical symptoms, wacky. Upstairs it worked fine. In her office it didn't. Is the power downstairs (on a separate circuit) dodgy? Was it the mains leads? No. It turned out to be the fairly old CRT monitor. Use a different CRT and the machines act normally. Plug in the old CRT and they start behaving erratically. The monitor was the last thing I thought of that would have been causing these problems.