What causes windows to reboot?

Any number of things can cause a random re-boot.

Bad or corrupt files.
Registry errors.

CPU, power supply, motherboard, ram over heating.

Loose ram, plug-in cards, power supply cables or drive connectors can also cause re-boots.

To determine if it is hardware, or a Windows issue, locate a DOS based burn-in or hardware test utility that will run off a floppy.
(I use Check-It.)

And set up a burn-in test.
If the hardware functions properly, then you have a software / registry problem.

It could be as simple as updating drivers, or require that you format the hard drive and start over.

Chas

What causes Windows to reboot?

Faulty programming from Microsoft????

Yes I'm a VMS bigot - where else can you get on a system that runs for YEARS without a reboot.

So, the answer is to switch to an OpenVMS system.
Even though this isn't the answer you want - this answer is also correct.

This has been a rare occurence for me. The XP reboot issue can be resolved by dumping XP Home and migrating to Pro. Just kidding. More often than not, spontaneous reboots are power related. Replacing the power supply was a good choice. In many instances, a good UPS such as APC and Tripp seem to keep voltage spikes and sags to a minimum while many of the Target Brands do little to remedy. I've heard that in rare cases, the PC's power supply can cause this if it is going bad or if you have too many drives hooked to it. If you are running more than 4 IDE devices on a 250W PS(or more than the molex connectors that come standard -- defeating it with a splitter) will cause this. In this case, I have recommended purchasing a 300W PS. Check your voltages from the PS.

If this happend after installation of a certain application, remove it and see if it continues. But I suspect it's power related.

Sounds like you could have several issues. I had a low-end out of the box computer that I gave to the wife and kids, and it had the reboot problem with XP home. I beat myself up for about a week in the evenings playing with it, and finally decided to re-install. Problem was still there. GRRRR. You know what fixed the problem? Reset the BIOS and reseated the memory chip. Works like a champ now. Hope that helps with the XP.

As far as W98, I would check to make sure that you have the power management update patch from MS, and then I would turn off power management. It is totally dorky on a desktop computer. (what was Bill thinking?) If you are running an older version of Mcafee Antivirus, check the scheduling of Virus def updates, it may be trying to install one, and with older versions, when this is finished, it reboots the machine to install the DATS fully. Also check task scheduler, to make sure the machine is not trying to do something, and run a scandisk, defrag, and a virus check. Also make sure the 98 is SE not original windows 98, which was a TOTAL nightmare for this type of stuff.