scw5741 -- Welcome to the AVS Forum.
It sounds like either you didn't get the right VGA to 5-BNC cable or you haven't plugged the BNC connectors into the correct sockets. Another possibility is that your VGA video card is not putting the Sync signal onto the right connection. The PE7700 manual does not seem to have adequate directions on the correct hookup.
I had the Toshiba MT700, a PE7700 clone. It had the DVI-I connector which I used from my PC video card's DVI connector.
Another alternative is to buy a VGA to Component cable and use the Component connections on your PE7700 (the Y, Cb/Pb, Cr/Pr inputs).
If you still have questions, you might want to contact BenQ.
BTW, the main "Official BenQ PE7700" thread is here:
http://www.avsforum.com/avs-vb/showt...ghlight=pe7700 . Several of the more experienced AVS members that have been active on that thread are Bronco70, Jeffcom, and miltimj -- see the last page of the old & very long PE7700 thread. One of those members may be better able to help you.