I need some help with my graphics cards!

I have two ATI Radeon hd 2900 XT's with cross fire. I currently have them pluged into my motherboard but I do not have the power cables pluged in, I'm just running my monitor off the mobo's graphics. But when I do have the power cables pluged in, window will power boot normally until I reach the home screen, which will show for 1-2 seconds then my monitor will lose signal. I have looked around for what could be causing this and have come up empty time and time again... So please help me!

These are my specs:

Windows 7 Ultimate
Intel i5-3570k 3.4Ghz
16Gb ram (Corsair Vengeance DDR3)
120Gb Intel SSD
2x 750Gb Seagate Barracuda ES.2 7200rpm
Asus P8 Z77-V LX motherboard
2x ATI Radeon HD 2900 XT
900W psu (not sure of the model)
HP Blackbird case

It sounds like the signal for the card is not the default. You will need to change this in Windows. Right now, it may be the integrated graphics port on the motherboard that is set as the default. When you log into Windows and Windows has finished booting up the startup programs and services for the logged in user, right click the Desktop and Left-click on Personalize.
In the left side of the menu should be the "Display" link. Left-click and on the left again there should be a link such as "Adjust Resolution" or "Screen Resolution" click on that and you should see "Change the appearance of your displays".

If you see multiple displays, you will want to use the 2nd one.
I use a Asus Monitor which is labeled as "2. VS278" and this is my external monitor (since I use a laptop).
Here is what my settings look like (Windows 8 however):
Display: 2. VS278
Resolution: 1920 x 1080 (Recommended)
Orientation: Landscape
Multiple Displays: Show desktop only on 2

If you have 2 monitors, you can use "Extend these displays" for the Multiple Displays setting. You will then be able to choose which display to use as the primary. I would use the display that is connected to the ATI card if you want that as the primary screen (this will be where the login screen shows up).

You can also use Win + P to "project" to another monitor, or in this case, a different port.

I suspect this "default" monitor setting is the cause of the loss of signal on your monitor.

I don't have a chance to change the setting after windows boots up. My monitors lose signal 1-2 seconds after the home screen appears. Also, when the computer is booting up, the windows logo animation does this weird thing where it blinks and then freezes then goes to the home page. I don't know if that means anything.

First you will want to connect the monitor to the default video port (this would be the integrated one). Once you switch the default to your graphics card, you can move the monitor's video cord to the video card.

Ok, I shut down my computer, plugged the power cables into the graphics cards, and plugged my monitor into the integrated graphics VGA port. When I booted up my computer, the monitor never even got a signal.

most motherboards will default to your GPUs once they are plugged in, just start up your computer and try all the ports you can, on both cards.

i have 2 HD6870's and when i first put my computer together, i only had 1, and even on the very first boot, it defaulted to my GPU, when i put in the 2GPU (just recently) it then defaulted to which ever one was on top.

so, dont worry about not being able to see, just leave it on and move the monitor cables around.

also, the resolution could be to high (not likely) to resolve this, start in safe mode, this puts the resolution at 800x600, the absolute lowest resolution from there you can figure out which port is the right one.

^^^^
the alternative it to just set you resolution to as low as you can, then restart the computer and try all your ports, you cant change the resolution in safe mode, so if resolution IS the case, you would still get a black screen on regular startup

also...that is a REALLY old card, the benchmarks i see are for FEAR

I tried that buy got the same results. It would boot up normally until the windows logo animation came up. About half way through the animation, it stopped and froze for about a second and then went to the home screen. The after a few seconds my monitor loses signal.

I am starting to think that it is a problem with windows because the logo animation doesn't do that when the cards don't have power. The same thing happened when I had problems originally installing windows. The only difference was that I never made it to the home screen...

Have you tried to boot up with just ONE of your ATI Radeon HD 2900 XT's plugged in? It could be a hardware issue (one or both of your GPUs), or perhaps an issue with Crossfire.

Try disabling Crossfire and booting with ONE ATI Radeon HD 2900 XT.
If the signal is lost again, try your SECOND ATI Radeon HD 2900 XT. Also try different PCI-E slots on your mobo if applicable.
If you still have no luck there, try booting with a completely different GPU (if you have one).

If you are able to boot with your mobo's onboard graphics, then I would assume you have a hardware problem or a graphics setting problem.