Dell Latitude D610 won't boot

Everytime I turn on my laptop it goes to the Dell loading screen just fine, then moves to a black screen with a flashing underscore in the top left corner and just... Stays there. The little lock with the 9 on the keyboard stays lit, and I can't do anything, but it seems quite happy to just sit there. If I press keys then nothing will happen for a while, but after enough keystrokes the keys will make a high pitched beep whenever pressed. I've tried a number of F buttons, particularly F12 (which didn't seem to help me) and F2 (which did nothing at all). A few start ups ago it went through what seemed like a test of the entire system, starting with coloured bars across the screen, and ending with the system asking me if I wanted to continue, or if I should put in the Dell device disk. As I don't have the disk I pressed continue and was brought back to the black screen.

The last thing the computer did before starting this cycle was to flag up a bluescreen which wouldn't go away so I had to hold the power button until the computer turned off. I think the memory might be full, but I can't get at anything if the computer won't turn on. Any suggestions?