HP does not support the nc6400 for Windows 7.
I did find a thread at the HP support forum that explains how to get your nc 6400 working with Windows 7. It appears as if you need to have a copy of Windows 7 SP1 in order for this to work on your system. Although you can obtain SP1 from Windows updates so that might work as well.
The instructions are very specific for this to work on your particular system. compaq nc6400 drivers for windows 7 - HP Support Forum