When you switch your computer on, go into the bios, browse around all the different settings and simply look for "enabled onboard graphics". When located press "page up" or down, to change the setting for it (to enable). Also, you might need to disable the AGP/PCI-E slot that you would have enabled when putting a new graphics card in, this option too is located within your bios.
If you don't know how to get into your bios, when you first switch on the computer constantly press down on either 'delete' or 'f12'. Bare in mind the key for your bios can vary depending on the motherboard you have. But those are the two most common I find.