Why does my computer not POST?

Yesterday I was working on overclocking my CPU/Graphics. i was succesful at first, but then as I turned settings up, the settings didn't apply. So I tried resetting my bios by taking out the CMOS Battery. After that the settings still didn't apply, but I don't care about that anymore. So I set all the settings back to default, but when I tried to boot, the windows 7 screen started to come up, but "froze" when it shows the logo, but it looked to me like the logo was still pulsing.

So I tried safe mode, that show'd me that the last thing on the screen to load was Disk.sys. Then I reset the CMOS again and still no luck. I tried taking out sticks of RAM, but the computer isn't even posting now. All the fans rev up, and lights come on, but NOTHING shows up on the screen. I tried taking out the card and re-installing it, that didn't work.

So now I have no screnn (post, or boot) So first, how do I get the screen to show? Second, how do I get windows to boot, thanks. One idea, maybe its the CMOS Battery is running out? The computer is a hand-me-down, so I would guess its a several years old.

Last night I reset the BIOS overnight while having all the RAM, Graphics, and Drives unplugged, that didn't work.


Specs:
ASUS P5n-E SLI Motherboard
Core2 Quad Q6700
4GB DDR2 800 RAM
XFX 8800 GTS
Corsair 525W PSU
Antec 900 Case
DVD Drive
Several Hard Drives