Inspiron 1720 display problem

and would like to learn more about this problem.

Laptop came in for malware/virus cleaning and replace the Mic/Headphone jack which was broken from it's mount and pushed in. Ordered the jack, and proceeded to do the virus removal then shutdown. When the jack came in, opened the laptop, replaced the mic jack, and decided since the cpu fan was dirty to go ahead and clean that and apply new heat transfer compound and replaced the heatsink.

Powered up laptop and no video on LCD screen.  PC boots up fine otherwise and no beeps are heard during post. Powered down and attached external monitor. On boot, external monitor works fine but still no display on LCD. Tried Fn F8 and no change other than the external blinks. Since external monitor works fine, we decided to order a new LCD assembly with new inverter from Dell. Installed that and still no display on LCD. tested and replaced cables to rule them out.

Updated Bios and drivers for laptop and no changes noted. Decided to do a full factory install from recovery partition, no change. For fun, I booted the laptop using a Ubuntu 10.0 Live CD and as soon as Ubuntu loaded, the LCD display was working fine.

I'm thinking that the video card drivers that load during post have somehow been corrupted or lost. I know that I'll probably have to replace the motherboard since it's an integrated graphics chip. However, can someone shed some light on what went wrong here? Why would the LCD not work at all during post OS load and loading of Vista while Ubuntu will run the LCD just fine. Is it a question of the video chips firmware? Can that be reloaded or shall I go ahead and replace the motherboard?

Thanks in advance for your input.