Precision 490 problem - two beeps and no video output

Hi everyone,

My Precision 490 won't boot or show any display. It just gives two short beeps and the numbers 1, 2 and 3 light up on the front. Does anyone know what this means? I've tried it with several graphics cards and still get the same result. Even removing the graphics card altogether has the same result.

Any help would be much appreciated. Thanks.