Mark,
As far as I know, XP Pro doesn't even support SNMP 3 in the Windows
SNMP service. There are third party packages that replace the Windows
SNMP Master Agent with their own SNMP V3 capable one, and then you
write sub-agents to implement your MIB using their tools.
Microsoft's direction seems to be WMI (CIM) for management. I don't
think that at this point they will release a v3 SNMP component unless
forced to. I could be wrong, however...
Desi