Monitors won't show boot screen or dos

I have a system with sli with two 7800gtx 512. The boot screen won't show with any monitor, the monitors turn on only when windows starts. Moreover if I open a command prompt and make it full screen the monitor switches off (meaning it gets no signal) and back on if i close it with alt-f4. This wasn't happening in the first few days, it started happening with no apparent reason. What can it be? Everything seems to work fine apart from this.
Daniele BrunengoIT Consultant, Web DesignerAsked:
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CallandorCommented:
This might be a Windows driver problem.  The displays that are not showing up are analog signals (DOS and POST screens), so try reinstalling the video driver to see if that fixes the problem.
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MarkCommented:
Which card do you have the monitor connected to. Also what motherboard do you have.
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Daniele BrunengoIT Consultant, Web DesignerAuthor Commented:
How can it be a driver problem if the monitor doesn't show post screens? The post happens before any driver is loaded. Moreover it happens with digital and analog monitors.
The card is the first sli card, xfx 7800gtx 512 Mb. The mobo is abit an8 32x.
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CallandorCommented:
> How can it be a driver problem if the monitor doesn't show post screens

True enough - I was thinking more of the DOS screen.  Does this happen with a single card running, or is it limited to SLI mode?
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MarkCommented:
There is a possibility that the pci slot rather than the PCIe slot is the  first inline to be initialized when startup occurs. The manual doesn't say which is the default value if the CMOS is cleared, but there are 2 choices in the BIOS. Have you cleared the CMOS jumper at any point before this happened.
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Daniele BrunengoIT Consultant, Web DesignerAuthor Commented:
Actually it happened with sli and without. But the problem was actually what sparkmaker said. I inserted an old pci card and could finally see the post screen... So in the bios I chose pci express as first inline and it worked. I don't understand what happened, I think I did clear the cmos at some point but if the first inline is not there the board should switch to the second... I have a soundcard on pci... Anyway it worked and so thanks sparkmaker and all you guys that helped me.
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