Onboard graphics

Please, please, please can anyone help me.
I have recently bought a pci graphics card, a gforce fx 5200 to be precise.
The only way of getting it to work is by disabling onboard chip in windows and also changing bios settings to disable onboard.
The only trouble is, when this is done i have no internet!
The usb modem dissapears from device manager completely and i get no internet at all, as soon as the changes are reversed, everything is fine again!?
The even stranger thing is that another computer networked to mine can recieve internet faultlessly!!??
Can anyone explain why turning off the onboard graphics can do this?

my mobo is a micro star ms-6714
and my onboard chipset is intel 82845G

Hm. The motherboard BIOS setup menu should allow you to automatically use a PCI or AGP add-on graphics if installed instead of the 845G integrated graphics. You don’t need to do anything else if it’s properly done. On this PC, I’m using a Matrox G450 AGP card on a 845G system. Nvidia and ATi cards also work well.