You did install the drivers for the new card I hope, and plug the monitor into it? If you did, then read on:
Most motherboards automatically disable the on-board video-chip when a new display card is fitted, but yours may be the exception. If it's not disabled it will cause a resource conflict with the new card.
You need to go into the BIOS settings by holding down the DEL key as soon as you turn the PC on. It may be a different key but usually it's DEL. If it works you'll get a blue screen with settings in white text.
There are several sections to the BIOS, each section listed on the first screen or page. Highlight 'Integrated Peripherals' and press ENTER.
Look for 'ON-BOARD VIDEO' or something similar, then press 'Page Up' or 'Page Down' to change it's setting from enabled to 'Disabled'
Press ESCAPE to get back to the first screen, then look at the bottom of the page to see what key to press to EXIT AND SAVE (usually F10)
If it asks "Exit and Save Changes? be sure to press 'Y' for Yes.
The BIOS setup will close and the PC will re-boot.