You are running Sophos Anti-Virus for Windows 2000+, with Application Control enabled. You have authorized Windows Live Messenger (WLM), but users find it is still blocked.
First seen in
Sophos Endpoint Security and Control
Windows Live Messenger (WLM) requires MSN Messenger and hence both must be allowed.
What To Do
You must authorize both Windows Live Messenger and MSN messenger if you want users to have access to WLM.
For more information on authorizing applications see the local product help or documentation.