Windows XP has by far been the preferred OS running in VMs; However, with arrival of Windows 7, IT admins have several important reasons. When users connect to a Windows 7 VM, the RDP7 protocol’ll be used to communicate between client and VM if RDP 7 or Remote Desktop Connection 7 (RDC7) client is used. RDC7 client is offered on variety of OSs, including XPSP3, Vista SP1 and Vista SP2 and same client is part of Windows 7 OS. When an RDP 7 client connects to a Windows 7 VM, it can take advantage of all new features implemented in Windows 7. However, when same RDP 7 client connects to a Windows XP VM, it’ll start talking 9-year-old RDP 5.2 protocol. To test user experience improvements that RDP 7 client provides when connecting to a Windows 7 guest VM.
Full Article: RDS blog