HD4870 no signal

Hi,

Today I bought a new asus EAH4870 which they installed in the store.
I come home and connect my PC with my VGA monitor.
The PC boots without problems and im able to login.
I still had to install the drivers so i did, but after a reboot im unable to get something on my screen.
Even the POST screen isnt visible, and my screen tells me it doesnt receive a signal. I bring it back to the store but there it works fine.
At home, the only way to get something on my screen is through the onboard VGA card.

Is the onboard VGA card in conflict with the 4870?
I tried to disable it in the BIOS but without success.

Anyone who has an idea what the problem might be?

GPU: asus eah4870
MB Foxconn G31MXP-K
PSU: 700watt