PC reboots at "Starting Windows"....what is this?

hello, this is very strange. I am on Windows 7 x64
Today I turned my computer on and it stopped at the "Windows Error Recovery".

From the 2 options given I chose "Start Windows Normally"....
After pressing Enter it will not go beyond the black screen where it says "Starting Windows" and inmediately reboots to the "Windows Error Recovery" again.
I then chose the other option which is "Launch Startup Repair", entered the Disk, wait for it to load system files but it reboots to the Disk loading the files again like an eternal loop.

Tried the F8 Advance booting options, chose "boot in safe mode with command prompt", it starts loading but then it reboots itself again!!

I don t know what is going on, this is very strange, yesterday I worked all day in the computer...

My system specs are these:

>XFX Nforce 750i SLI
>Intel Core 2 Quad Q9400 @ 2666 MHz
>3x2gb Corsair CM2X2048-6400C5 DIMM DDR2 PC2-6400U DDR2-800 (5-5-5-18 3-22-6-3)
>ATI Radeon HD 4300/4500 Series
>WD Caviar Blue 500gb


Thanks a lot