The IP address XXX.XXX.XXX.XXX you have entered for this network adapter is already assigned to another adapter Name of adapter. Name of adapter is hidden from the network and Dial-up Connections folder because it is not physically in the computer or is a legacy adapter that is not working. If the same address is assigned to both adapters and they become active, only one of them will use this address. This may result in incorrect system configuration. Do you want to enter a different IP address for this adapter in the list of IP addresses in the advanced dialog box?
In this message, XXX.XXX.XXX.XXX is an IP address that you are trying to set and Name of adapter is the name of a network adapter that is present in the registry but hidden in Device Manager.
I tried Microsoft's suggestion in this article "http://support.microsoft.com/?kbid=269155" however it didn't work for me. I had to create a "system environment variable" and give it the value of "1". Once that was completed I was able to follow the rest of the article. I removed all hidden adapters except for the "RAS Adapter" and I was then able to add the new IP address.
How to create a system variable:
On a Windows machine right click "My Computer" --> Properties --> Advanced tab--> Environment Variables--> System variables -->Click new --> Specify a "Variable name" (devmgr_show_nonpresent_devices) and the "Variable value" which is 1