What i think you need to do is to enter the bios and disable the onboard graphics, otherwise each time it reboots the computer installs the software again for the onboard vga.
uninstall the graphics (onboard)in add/remove programs), change to svga in control panel/display then shutdown your computer.
insert the new graphic card and restart your PC.
you will also of course have to change the monitor cable over to the new graohic card connecter.
you should be orompted to insert the driver cd to allow installation of the new card.