This tip may help you.
We use virtualization A LOT on our environments, as a symbol of trust to this concept we only have half of our web production servers as real machines, the other half is virtualized.
A common issue we found is that after installing SP2 on a Windows 2003 Ent/Standard installation or simply after installing certain security updates on Windows 2003 Ent/Standard R2 installations we lost sometimes the ethernet cards of the Virtual Machines (VM).
Well, the solution is quite simple, this problem usually only happens when you have not installed the client VMware tools, after installing them and restarting the Virtual machine you will be back in business.
Strange? yup. Drove me nuts a couple of times.