I ran into an issue recently on a 64-bit Server 2008 R2 machine running on VMware.  The problem started after trying to do a Multicast Windows Network Load Balancing setup.  The system would constantly hang or would freeze after making ANY network changes.  I found Aaron Saikovski’s blog and the steps that he outlined seemed to solve the issue.

Basically, you need to make sure that when adding the network adapter to the virtual machine use one of the VMXNet virtual devices and not the NBT100 or E1000 devices.

Aaron’s blog post: http://blogs.msdn.com/aaronsaikovski/archive/2009/09/13/windows-server-2008-r2-nlbs-and-vmware-workaround.aspx