So you are using a new Windows 7 installation and developing a Windows Mobile application, and you have been using it just fine on another dev box or in an earlier version of Windows. You deploy your application, and it needs to communicate over the internet, so you go into the Network tab of the Emulator Properties and enable the network adapter.
Only this time, it comes up with an error message about installing Virtual PC 2007.
And then you find out that Virtual PC 2007 does not play nicely with Windows 7.
Well it all has to do with Virtual Machine Network Services. Here is a blog post from Brian Peek on how to solve the problem:
Windows Virtual PC and the Microsoft Device Emulator