By default, computers that are running the Windows Vista and Windows XP operating systems cannot access RemoteApps and Remote Desktops that are published by Forefront Unified Access Gateway (UAG). However, on computers that are running Windows XP with Service Pack 3, Windows Vista with Service Pack 1, or Windows Vista with Service Pack 2, you can apply a Remote Desktop Connection 7.0 client update to allow these computers to access RemoteApps and Remote Desktops.
To enable RDS on Windows Vista and Windows XP
Download and install the update described in Knowledge Base article 969084: Description of the Remote Desktop Connection 7.0 client update for Remote Desktop Services (RDS) for Windows XP SP3, Windows Vista SP1, and Windows Vista SP2.
On computers that are running Windows XP SP3, you must also enable Network Level Authentication (NLA), by turning on the Credential Security Service Provider (CredSSP). For more information, see Description of the Credential Security Support Provider (CredSSP) in Windows XP Service Pack 3.