If you have been having problems with it for a while, Win 7 may have just aggravated it.
Do you think the keyboard is working OK, cleaned it out lately?
That keyboard shortcut may be a Dell thing and you may have to load some Dell software to handle that. Do other buttons like the eject DVD or whatever working normally?
Have you tried a different external monitor? Someone with a 10 year old monitor said his went black during boot and stayed that way, but came back after putting the computer to sleep and waking it up. Replacing the monitor fixed the problem, and his problem was related to Win 7, or so he thought.
Maybe try turning off Aero to see if it helps.