virtualbox.org

Current version of virtual box 4.1.12
Mac OSX 10.7.5
Windows 7 Enterprise virtual machines with VDI File
Network Adapter using on windows VM: Intel PRO/1000 MT Server (NAT)

What can I do to improve network performance on windows VM?

Your question is very vague and you do not explain if you are asking for advice on how to optimise the speed of if your question is related to a specific problem.

So for now I can only tell you this, since you give no information and context : use Bridged with Virtio adapter.