Net send

Companies start to use this standard feature of Microsoft as a new way
of spamming other people' machines, unasked, and with almost no way to
communicate back or to find out who is sending you the rubish.

An example of a company abusing this way of messaging is : Broadcastadvertiser

But more important, how to get rid of this kind of messages.

1. Disable the Messenger Service.

2. Block NETBIOS and RPC (135) at firewall level.

Please be aware that a 'spammer' or other persons might send 'faked' messages.
It is quite easy for them to use default system variables to show your user-id
or computername in the messages.

Read more about it in the following microsoft article:
Microsoft Knowledge Base Article - 330904