Vista - Remote assistance from Vista to XP

I am working in an enterprise environment and we are unable to offer remote
assistance from Vista Enterprise to XP Pro SP2 systems.

I launch msra /expert and try to connect. Remote assistance identifies the
remote user but then generates this error followed by some suggestions: "Your
offer to help could not be sent"

The event generated on the helper system:

Log Name: System
Source: Microsoft-Windows-DistributedCOM
Date: 11/16/2007 8:55:10 AM
Event ID: 10006
Task Category: None
Level: Error
Keywords: Classic
User: N/A
Computer: XXnameremovedXX
Description:
DCOM got error "2147746132" from the computer XXnameremovedXX when
attempting to activate the server:
{3C3A70A7-A468-49B9-8ADA-28E11FCCAD5D}


No obvious events are generated on the remote system. DCOM permissions
appear to be set correctly, but maybe not. Anyone seen anything like this?

On a related note, if I try to initiate a RA session to a XP systeem with no
logged in users, I get a different error:

Log Name: Application
Source: Application Error
Date: 11/16/2007 9:19:39 AM
Event ID: 1000
Task Category: (100)
Level: Error
Keywords: Classic
User: N/A
Computer: XXnameremovedXX
Description:
Faulting application msra.exe, version 6.0.6000.16386, time stamp
0x4549adda, faulting module msra.exe, version 6.0.6000.16386, time stamp
0x4549adda, exception code 0xc0000005, fault offset 0x00029e7b, process id
0x1554, application start time 0x01c8286bfe815236.