My computer started doing this a couple of weeks ago. When I power it does not send a signal to the monitor. I have removed all internal parts except for the PSU, the processor, RAM and vidoe card and it still does the same.

PSU - OKIA 450 w (new)
CPU intel dual core
RAM 1 gig
Video - geforce 6600LE 256 mb
MOBO - p4m800pro-m2 rev2.0

I replaced the PSU. it now has a OKIA 450 watt PSU and reseated the RAM, CPU and video card and still the same. The vidoe card requires an additional Molex connection for power. I found that if I disconnect this then the computer, monitor and video card are operational however when i loaded the driver for the geforce card it did state the Molex connector was not connected so the card was lowering its performance due to this. When I reconnect the Molex connector on the vidoe card, everything hangs up again and no signal to the monitor.

this mobo does have on board vga. I tested that and it works fine. any idead on the graphics card issue?

Thnaks

Time to change Video cards... assuming you have already downloaded driver updates.