Computer continually restarting; will not boot

I have a computer I've had for a couple days now and it has been working now until recently. A friend of mine offered to sell it to me and I told him I'd check it out before buying it, so the most I've done to it up until recently was plug in a mouse, external and keyboard and search around it and run things off of the external.

Today I plugged in my USB harddrive, keyboard and speakers to the computer to play around with it. It popped up with the old, "new hardware has been installed, for this to take full effect we need the machine to restart, or you can do it later" or whatever so chose to restart the machine, it hasn't said this in the past when plugging in the hardware. After restarting, the machine will not properly boot up and is stuck in a boot loop. It'll power on, light up and the fans will kick in but after about 8 counted seconds the system will restart and try to boot up again. The harddrive and CD drive will make noises and the keyboard will light up but the monitor doesn't display a thing. I cannot get to the bios nor can I boot Memtest, or absolutely anything for that matter. At a complete loss on what's going on.

To my knowledge the hardware this machine is packing is;

MSi x48 Platnium Motherboard (MS-7353)
Intel Core 2 Duo E8200
Radeon HD 4890 1gb
650w Corsair Power Supply

The system is running Windows 7 Professional and the friend who's owned it has had it for almost a year now without any real problems. Anyone have a clue what's going on? I'm at a complete loss on how to diagnose this or what it could even be at this point. Of course I've unplugged the external, etc when trying to boot in case that was interfering as well.