From Russ Kaufmann's blog:

"Since the issue keeps coming up, I have decided to add it to my blog.

Many people like to create a basic VPC or VS image and then copy the vhd and vmc files and rename them to build a new virtual machine. Sysprep is often used so the copied image isn't exactly like the original. It sure works nice, except, the ethernet address is not changed by sysprep and TCP/IP has lots of problems when there are multiple systems with the exact same MAC.

The answer is to go into the vmc file (make sure the virtual image is not in use) and remove the hex code from the xml tags. Search for ethernet_card_address and then remove the information between the opening and closing tag.

When you restart the virtual machine, it will automatically generate a new random address so there will be no conflicts."