Check for a item in BIOS to disable the onboard video adapter, or check the computer's or MB's documentation for a MB jumper to do the same.
If that is N/A, then:
1. Right click on the Desktop and select Properties.
2. Click on the Settings tab, then the Advance button.
3. Click on the Adapter tab, then the Change button.
4. On the first Wizard screen, hit the Next button.
5. On the next screen, select "Display a list of all the drivers......", then hit Next.
6. On the next screen, select "Show all Hardware", then highlight "Standard Display Types" at the top, then in the right window, select "Standard Display Adapter VGA", then hit Next.
7. You may be asked to insert the 98 CD to load the driver.
8. If Windows asks if you want to restart, say NO.
9. Shut down the computer and install the new video card.
When you reboot, Windows should detect the new card and you can install the drivers for it.
If no luck, then:
Right click on My Computer, and select Properties/Device Manager.
Click on the + next to Display Adapters.
Double click on the onboard video adapter.
Check the box for 'Disable in this hardware profile', click OK, and re-start the computer if it prompts you to do so.
After it reboots, shut it down, and install the new video card.
Move your monitor over to the new graphics card, restart, and install the card's drivers.