Some information about Active Directory Schema for Personal Virtual Desktop Pool
Personal virtual desktops do need a Windows Server 2008 or Windows Server 2008 R2 schema. What does this mean is, you need to upgrade your schema to at least Windows Server 2008.
Here are the Active Directory requirements for personal virtual desktops:
To deploy personal virtual desktops, your schema for the Active Directory forest must be at least Windows Server 2008. To use the added functionality provided by the Personal Virtual Desktop tab in the User Account Properties dialog box in Active Directory Users and Computers, you must run Active Directory Users and Computers from a computer running Windows Server 2008 R2 or from a computer running Windows 7 that has Remote Server Administration Tools (RSAT) installed.
You must use a domain functional level of at least Windows 2000 Server native mode. The functional levels Windows 2000 Server mixed mode and Windows Server 2003 interim mode are not supported.