|Supply as many details as you can come up with !|
Tell us the make and model of your brand name system, or if you have a generic desktop system, the make and model of the mboard.
The specific model of a brand name system is shown on a label on the outside of the case somewhere, or it can often be determined by going to the brand name's web site.
The model, sometimes the make, of a mboard in a generic desktop system is usually printed on the mboard's surface in obvious larger characters, often between the slots.
The model is often also displayed on a logo (graphical) screen early in the boot, but it's often not as specific as the specific model number.
For Dell computers, they have a Service Tag number - the specific model can be determined by using that on their site, or can often be determined there automatically by you downloading some software. The Service Tag number should be on a label on the outside of the case, probably on the bottom on a laptop, on the back on a desktop, and is often also shown in the bios Setup.
Are you getting video before Windows loads while booting?
On the monitor or on the computer?
If on the monitor, if it can be connected by DVI or VGA, usually you can use one or the other, not both - if you still have the DVI connected you probably must disconnect that connection, when the computer is not running.
Monitors are NOT "hot pluggable" - never unplug or plug in the monitor to the computer when the computer is running !
If the monitor is DVI, there is no such thing as a DVI to VGA adapter that can be used on it's DVI connector to convert it so it works when plugged into a VGA video port, that works properly for the monitor - there are not enough connections in the VGA port for that to work.
If on the computer, read on.
If you have more than one video port, e.g. 2 on the video card in a slot, or 2 built into the mboard, try connecting the monitor to the other port - one port is Primary, the other Secondary - sometimes the video does not work properly if you connect a single monitor to the Secondary port.
If your desktop is fairly recent, I have noted that some recent mboards have onboard video that has a type of DVI port or ports that cannot work properly if you try to use a standard DVI to VGA adapter with the DVI port so you can connect a VGA monitor to it.