I know a lot of people using XP Mode to solve app compat issues as they move to Windows 7. It is simple enough to deploy and great for the few user scenarios where you don’t want to deploy MED-V. Managing the XP Mode VMs is an important task and the easiest way to manage it, outside of deploying MED-V, is to join it to the domain. That way you can manage it the same way you manage all the other machines in your domain. Joining the XP Mode VM to the domain is done the same way as any other machine but there are a few permissions that need to be configured to get it to work properly once it has been joined.
Because the local user context will be changed from the default local account in XP Mode to a domain account you will need to add Domain Users (or the specific domain user or users) to the Local Users group on the XP Mode machine.
And because the users are connecting via Remote Desktop Protocol (RDP) you will need to add Domain Users (or the specific domain user or users) to the Remote Desktop Users group on the XP Mode machine.
With those permissions set your XP Mode machine will be joined to the domain and your users will still have access to the VM with their domain credentials.