Ok. I have recently installed Mandriva 2006 on a Dell Optiplex that has been running Mandrake 10.1 for well over a year.

This machine was built early last week and has been running GREAT!

However, something happened last night and when I came into work today the machine had been rebooted and was at the command line login prompt. I figured oh well, and logged in and started up xwindows. The monitor light flashed green to orange and back a couple of times and then the machine rebooted!

I can't start xwindows now. I have looked through the messages and syslog log files and don't see anything odd. Where else should I look to see what the problem is?

And has anyone else had this problem before?

When I left last night, xwindows was running.

I've had had this problem but I can't say if your problem is related.
I have a vey old machine and after installing NVIDIA drivers starting X made the comp reboot.
I fixed the problem by disabling some AGP "enhancements" in the BIOS.