Endless rebooting problem - also, impossible to boot from DVD Windows disc
 

Help! My computer (newly built 3 months ago) suddenly doesn't want to start and reboots endlessly.
Yesterday eve, I just put it on "sleep mode" for the night (as I had to finish reading few more issues on some Internet sites) and when today wanted to continue, it just got frozen. I pushed the restart button but it went to endless rebooting. No way to get in Safe Mode (F8), the black prompt screen blinks very shortly only once and goes to reboot again. Going in the Setup BIOS menu (Del) doesn't change anything. No way to restart from the DVD drive with the Windows installation disc neither. It just doesn't respond to ANYTHING.
That's beyond all my imaginary limits... Could anyone come with a suggestion?
Thanks in advance!

Specifications:
PC/Desktop
Windows 7 64bit
CPU AMD FX-8350 8-Core 4GHz AM3+ 16MBCache
Motherboard ASUS sAM3+ Sabertooth 990FX R2.0
Memory DDR3 8GB(2x4) PC 1866 CL9 G.Skill RipjawsX
Graphics Card Gigabyte Radeon GV-R726XWF2-2GD
Hard Drives 2x Seagate Barracude 1TB 3.5 inches SATA3 64NB Cache 7200rpm