Computer starts from main switch and the fan speed goes insane, help?

I recently purchased DDR-2 533Mhz 512mb x 2 RAM chips for my computer, I know what I am doing and how to insert them correctly etc. The computer is a dell dimension 8400.

I have currently DDR-2 400Mhz 512 x 2 for RAM which occupies 2 slots. I read that if you combine 400mhz and 533mhz that they would still work fine and the 533 would be used as a 400 one or so.

I switched on my comp from the mains switch and it started up by itself. The Fan then started at a normal speed and then just increased and increased, i panicked and tried to hold the power button at the front but the fan kept going faster and louder, so i switched it off from the mains switch.

I opened the computer up and removed the new RAM chips and kept my original ones. However now the problem persists, the mains switch starts up the computer, and the fan speed is very worrying, the start up does not display on my monitor either.

Please can anyone help?