After I installed XP Pro my computer would randomnly reboot itself in the middle of all sorts of programs about every 1 to 1.5 hours. I reloaded XP, loaded SP1 from the update site, checked the CPU fan, unchecked "automatically restart" under my computer/properties/advanced/startup and recovery settings. None of these worked so I went and had the power supply replaced. This fixed my first problem and now the system no longer reboots by itself. Instead, the monitor shuts off and the system does not respond. I can hear the CPU fan running, however my monitor is shut off and the power button no longer functions (even if I hold it for a few seconds) and I have to toggle the main power switch at the back to get it running again. Now, instead of crashing/rebooting every 1 hour or so, it will work for as long as about 8 hours before the first crash/shut off. I have tried reformatting my hard drive, reloading XP Pro, with and without SP1 and it still happens. Any ideas on how to fix this or run further tests to see where the problem lies would be greatly appreciated. Thanks.

When the monitor shuts off and the system doesn't, it usually means that the video card has switched to a mode that is not supported by the monitor. What video card are you using with what drivers? Your monitor may also need its own set of drivers so that the available video modes will be restricted.

Eric's suggestion is probably your best bet. Have you tried looking in event viewer? They may be an entry written around the time of the crash/reboot that could help to identify the cause of the problem

I am haviung the same problems with XP but not the monitor problems, and its doing it with and without SP1 - computer will just randomly just do memory dumps, some go to blue screens, some just restart the computer im running Soyo Mobo SY-K7ADA PC2100 DDR 128MB Ram 10gb Segate 60bg Maxtor 64mb Jaton GeForce2 PCI video card Phillips CDRW Sansung CDRom its done it a few times, i havent reinstalled the OS yet, but I have service packed everything, XP and IE6 - still getting iexplore errors and hte memory dumps. Event viewer just says that a memory dump was done, thats it, no reasoning why or nothing, and I cant figure out whats causing it, if its the OS or the hardware... Any ideas?

Test the memory using memtest86.

Well, try getting Norton System Works 2002 or 2003, any earlier versions won't work on XP. Run the Windows dianostics. It may be due to bad registries that will cause your computer to act in a wierd way. You did state that it AFTER you moved up to XP Pro. Another way to solve this problem is that instead of upgrading to XP Pro, you will have to do a clean install, that should get rid of most of your active registries. It will also mean that you will have to do a new install of most of your programs but that's better than dealing with a randomly crashing computer, right?