Although it doesn't appear like the monitor is at fault, try swapping in a known good monitor to see if it works normally. If not, and because you can't reach a screen where you see a basic Windows display, it appears like it's time to perform a reinstall of the operating system.. If you've got the Vista reinstall disc, which Dell normally provides with their systems, plus the driver/resource CD, you should be able run a repair install using the Vista disc.. A repair install should allow you to reinstall the operating system without damaging your personal files.. If the repair install doesn't fix the issue, then a full format and reinstall of the operating system will be necessary.. Remember to install the drivers from the driver/resource CD after the operating system is finished installing.. Once that's done, you can use your backups for replacing important, personal files, pictures, etc.
Hope this helps.
Grif
Dell xps desktop with Vista 64 bit. Microsoft Office 2007 was not working, so I reinstalled. The CD/DVD drive stopped reading disks a few weeks ago, so I installed office 2007 from microsoft website using the product key from the original cd. Download appeared to go fine, but when I rebooted to finish the installation the monitor went black after startup. Computer sounds fine during start up and monitor shows dell logo and microsoft windows flashes accross screen while loading. Screen goes black where you would normally see the desktop. The curser is visible on the black screen and moves around screen when you move mouse. I have tried safe mode, repair from boot menu, and last good configuration and had same result for all. Any other ideas would be appreciated.

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic