Sounds very much like a fried GPU.
I had a Quadro NVS 135m go just 2 weeks ago, succumbed to the heat/fabrication problem on the 8-series GPUs. I could boot into windows XP in safe mode, but the screen would go blank on a normal login immediately after the XP loading screen.
The PC uses the standard VGA windows driver for safe mode, and also during the windows load screen, then the screen flickers and switches to the graphics card driver on a normal boot up. Sounds like your PC is happy using the vga driver, but the moment you try to go through the graphics card you're in trouble. Almost certainly the card is dead.