I am trying to install the D-Link DFE-580TX 4-port NIC card on my Windows 7 x64. However, they only have drivers up to win NT4. I was wondering if the Win XP compatibility mode in Windows 7 will help.
Sorry, but no. Compatibility mode is for application software, not hardware drivers. Not even running XP in a virtual machine will work if you need hardware drivers (but certain "soft" drivers such as printer drivers sometimes work).
If you can't find 64-bit Windows 7 or Vista drivers for your NIC then you'll be out of luck.