Yes, You can sucessfully discover Win XP with OnCommand Balance. Often, you need to either disable Firewalls or open the required ports as specified in many KB articles. Also in VDI environments, Users will typically disable collection alerts if the VDI environment is continually powered on and off (Under Admin-> settings --> Email ->Alerts )
As Peter mentioned, Win XP is not an officially supported "Server" based platform.
Best regards,
Don Bourque
Insight S&A