"Searching for signal" on 3400MP

Hi everyone, I've been having a few problems getting my desktop to display on my 3400MP. All it keeps saying is: "Searching for signal" on a blue background. The odd thing is, the projector works on other laptops! I have updated GFX and system drivers, and also tried to enable another display in the ATI control panel, but to no avail (I have also been trying to "swap" between the two displays by pressing fn + F4)

The projector's connected by a 15 pin cable and by USB and the laptop has ATi Radeon xpress 200m onboard graphics and the laptop is Compaq nx6125.

Many thanks in advance  :)