I'm running Windows XP and everytime my PC boots up it automatically reboots. It will not load the desktop and if I try to access it in safe mode it does the same thing. There is an option to disable the Automatic reboot from the F8 menu but that does nothing. The PC still reboots automatically. I've also disconnected the CDROM, the second HDD and replaced the memory with no luck. I have no problems rebuilding the PC but the option to change the boot order does not appear either and I've tried hitting the following keys...ESC, F2, F8, F10, F12 to see it the setup window would appear. I think I’ve exhausted all options but any input from anyone would be greatly appreciated. Thank you.

Try F1 or DEL. Sounds like your CPU might be overheating. If you can get into the BIOS, check the temp.

Thank you Jam. The F1 and Del keys didn't work at first but after unplugging the floppy drive from the Motherboard it caused the PC to prompt me to select F1 for setup or F2 to continue. I don't know why it worked that way b/c I unplugged almost ever component earlier and nothing happened. Anyway...I was then able to get in and change the boot order to the CDROM. The problem has since then been resolved. Thank you again.