Most network administrators don't really care if the users waste their time chatting away on the net. It's not their job to make sure that users are productive. That is up to their respective department managers to decide.
However, for most administrators, P2P programs such as Windows Live Messenger are security risk as it has the capabilities of files transfer. With that, any form of files, inclusive of damaging scripts etc can get into the network.Users can always claim ignorance but network administrators do not have that luxury
If you are using squid as a proxy and would like to block your users from using Windows Live Messenger, you can take the example from the following ACLs
# Windows Live Messenger
acl wlm_mimetype req_mime_type -i ^application/x-msn-messenger$
acl wlm_urlregex url_regex -i gateway.dll
http_access deny wlm_mimetype
http_access deny wlm_urlregex