Motherboard/Processor help

Hello all techies,
I have a Dell Dimension 4550 with the original mobo still installed. I had processor disaster in so much thta i removed the processor and heatsink in order to clean off the dust, however i hadnt at this time removed the power cable! After refitting the processor my pc wouldnt start at all. So i have purchased another processor ( a P4 with HT tech 3 GHz 800MHz FSB 1MB L2 cache) fitted it and a new fan after remembering to remove the power cable this time! This time when i press the power button my pc works, kind of lol. There is power to the mobo coz the fans r working but no post and no video output and no hard drives whirring. The funny thing is when i remove the ide cable for the hard drives and power up the pc they are working! So there you have it thats about as much as i know at the moment have tried the memory test fine processor test finehard drive test fine (except the ide cable problem), What else shoud or could i try? Any help greatly appreciated as my back up computer is like me, old and slow lol. Fanx in advance