Application Sharing error in Messenger 4.7.3000

Two Questions:

1) Application error:

Normal messaging operations OK but 'Start Application
Sharing', either as an initiator or receiver, causes an
error a few seconds after the exchange of invitation and
acceptance messages. Messenger does not fail on
the 'other' PC.

The error is: "Windows Messenger has encountered a problem
and needs to close. We are sorry for the
inconvenience." .. etc.

The error signature is:
AppName: msmsgs.exe
AppVer: 4.7.0.3000
ModName: unknown
ModVer: 0.0.0.0
Offset: 00000000

I am running XP Professional 2002 Service Pack 2 and
Windows Messenger is version 4.7.3000.

2) Capacity

How many other Messenger users can I share applications
with? Either a hard limit or practical limit?

Thanks