entering the bios differs from manufacturer to manufacturer,it should say on you screen which key to press when you first turn on the computer,you will need to be quick though.
if may say something like "hit F1 to enter set up" or similar.click here for the most common keys to press.
this is what i did to disable my onboard graphics with xp.
first,if you have ever updated your onboard graphics driver in the past,remove it from add remove programs if its listed here.
next,click start/control panel/performance and maintenance/system/hardware/device manager and look for "display adapters" or in "sound video and game controllers" for your onboard device and right click it and select "uninstall"
when/if promted to restart the computer,select "will restart later" and instead shut down/turn off the computer.
next open up your case and insert the new card.turn on the computer and as soon as it starts enter the bios straight away and disable the onboard device,dont let windows start.
i have an ami bios and i disabled it under the "advanced/chipset" menu.
save and exit the bios(for me this was F10) and when windows starts up it should detect the new card and prompt you to install the drivers.either point windows to your cdrom drive or click cancel and download and install the lastest driver from nvidia`s or ati`s website.