continous restarting on motherboard screen

Alright so one day I left my computer on and when I came home it was continuously restarting right after the motherboard screen. I've had no issues with this computer previously and am puzzled how to fix it. I've tried putting the windows disc in and trying to boot from that to restore but when I press the key to get into the bootmenu nothing happens. None of the other options work either.

I'm thinking it's some sort of hardware issue, possibly the HDD.

anyone have any idea?

The first task is to figure out whether it's hardware, obviously.

Try going into the BIOS at boot-up and just sitting there for a while. If that fails to work also, or it reboots just as readily, the hardware is definitely cactus.

Otherwise, if you can successfully sit in the BIOS as long as you like, try changing the boot sequence and boot from something else - anything at all. An old DOS floppy, or a USB stick with another OS installation, or even a non-bootable CD. The aim is to figure out whether the same rebooting behaviour is evident.

If not, it looks more likely to be software. A damaged main OS installation, to be precise. Try pressing F8 at startup to go into safe mode. If that works fine, download the "Autoruns" utility from the MS website and use it to start selectively disabling startup items in the hope that the rebooting will stop in normal mode too.