|On a mboard with onboard video, your bios will often NOT autoselect AGP when you insert an AGP card - there is a setting in the bios somewhere to initialize AGP video first (default is often PCI) or to initialize a video card in an AGP slot first(default is often onboard video, whatever label they give it). There may also be a setting to assign an IRQ to AGP, but that usually isn't there in more recent bioses - it is assigned automatically.|
If you don't do that, on some mboards you will get video (VGA video) until Windows starts to load, then the screen goes black and stays that way - Windows loads anyway, unseen. On some mboards with XP on them, you will get still get video in Windows but it's SVGA video, not AGP video.
Your Display in Windows should be set to a basic mode such as PCI VGA before you install the drivers for the card.
"I've tried to disable it from the motherboard and the bios.
It doesn't work----- I'm to understand that this particular motherboard has NO jumper for disabling the onboard card. "
There is usually no setting to disable the onboard video in the bios, with the possible exception of a setting I've seen on some Intel brand name mboards like the second example in the first paragraph. I have not seen any mboard that has a jumper that disables the onboard video since mboards for 486's. Usually the onboard video is disabled automatically by the bios when you plug in a card to the primary video card slot, in this case the AGP slot (although it may not be in this case if you had plugged in a PCI video card).
You can have no video at all in a situation like yours, assuming your card is properly plugged into it's slot and you have set the bios settings as above, and the monitor is plugged into the card, not the onboard video port, if the video isn't finding the monitor as a P&P one. With some cards, the monitor must be on before you start up the computer or the video won't find the monitor properly - if you power up both at the same time, the video doen't "know" what settings to use. With those cards, if you then press Reset and boot, the video will be fine.
Despite the blinking led on the monitor, you can often press F8 repeatedly while booting, you will then get the basic VGA video all cards use in the initial stages, and you can then select Enable VGA mode from the Safe Mode menu that appears.
When you get to the Desktop, go to Display - Settings - Advanced - Monitor - Properties - Driver - Update Driver - Install from a list or specific location - Don't Search - choose Plug and Play Monitor, or if you have the drivers for your monitor handy, Have Disk - point it to the location of the drivers, pick your model.
Then reboot normally - your video should be fine.
Take a look in Device Manager - Display Adapters. There should be no listing of your onboard video, with the possible exception of it being there but it is disabled because you disabled it before you installed the AGP card.
If there is a listing for your onboard video there that is not disabled, that means the mboard and the bios did not auto disable the onboard video when you installed the AGP card - Disable it (Un-installing it will not work - it will come back next boot).
If there is a listing there for your onboard video there that is already disabled, usually you can Uninstall that entry and it and the onboard video will not appear there thereafter.