(Redirected) T5400 won't boot

My T5400 will not boot up. (my previous post on this seems to have gone)

History:

First; Fan occasionally ran flat out; reboot solved that.

Now; the machine does not boot up at all. I have number 3 and 4 lamps on, and a very rapid series of blips in sets of five. Dell trouble shooting does not mention 5! 

I have cleaned out all the dust, nothing seems damaged or loose

If I push the small power test button on the back the lamp goes green.

If I push the button while turning the PC on I get no beeps, still get 3 and 4 lamps.

Any help much appreciated.