OK so
1) plug your monitor into the old connector on the mobo.
2) un-instal the driver base for the onboard graphics - Start > Control Panel > Add remove Programs.
3) reboot & hit Del at the drives detection posts
4) go into BIOS (basic input output system) and turn off the onboard graphics (check manual for where)
5) F10 and Yes to save bios settings
6) At this point you will have no monitor output on reboot so just turn your machine off (press & hold button for 5 sec)
7) Plug monitor into Graphics card and Start up PC

You should have basic low res display and when windows posts it will do the "Found new hardware" routine so follow the prompts
good luck