looking for shareware/freeware (aka no price!) of a program that can message computers on the network without them requiring the program to be installed too!!
Generally, both parties need to have the software installed so that you have something to share the connection over. There are quite a few different pieces of software available.
That can be used to message any PC connected to the network. This isnt considered remote control as that would allow you to control which you would use UltraVNC or Remote Desktop for.
If you didn't already know, you use netsend from the command prompt.
Unfortunately on some networks (like my college) NetSend is blocked, in which case you'll have to keep on looking