If HP's site doesn't have what you need, you can find the drivers through these alternatives: HP Compaq dc7700 Small Form Factor PC
The long answer: You can use a modified or generic designed for Windows 7, or you can use the built-in Windows 7 driver that provides basic functionality. However, to unlock full resolution and Aero Glass, a specific workaround is needed. hp dc7700 graphics drivers windows 7
Although it has a standard VGA port, the motherboard features a specialized PCIe x16 slot designed specifically for a "DVI ADD2" adapter. If HP's site doesn't have what you need,