to uninstall the previous drivers, go to Add/Remove Programs and select your nvidia or ati display driver for the integrated graphics adapter. the uninstaller should remove the display driver and then you want to restart the system for the next step...
as soon as the computer is starting to boot up, you need to strike a key in order to enter the system BIOS. this is commonly the DEL or one of the F# keys; you may need to consult your system's manual for the key but it may show on the POST screen... under enter setup or similar.
once you enter the BIOS, you'll need to find the control setting for the onboard graphics adapter. this is probably located in the "integrated peripherals" section or something like that. you should be able to find a toggle vga adapter types or an option to disable it. once you've done that, you can Exit and Save changes to the CMOS and the computer will restart (you can shut it down before it loads OS and install the new card).
you should now be able to install and use the new graphics card, post back with your results...