i wanna send a message to a laptop which runs windows lives next door, what should i do? should i know his ip or something else? how can i do this? thanks!

what kind of message?
you can sent a mail, icq, IM....

linpopup may be what you want. It's a port for linux of the winpopup protocol. Your friend needs to be running winpopup on his/her machine.
cheers,
jdk

As already stated we need more information.

I assume the computers are not on the same local network. Windows messenger service (net send command or winpopup) should be disabled since it is a security risk. You also do not want to expose your system by opening SMB ports to the world. In fact is no longer supported in Windows Vista.

You can use one of the instant messaging services are maybe:
http://sourceforge.net/projects/pnim/

thanks for your reply. i am new to linux, and maybe new to computer, i just learn computer by myself. i do not know as much as you guys. i know i can use some special software, like IM which i am using daily. but yesterday i came up an idea that how to send message without special IM software?

maybe send a "hey you, have a good day" message to the guy whos lives next door. i just know he runs windows system or maybe Mac os,but linux,and i think we are on the same local network.because we live in dormitory. it is compus network,right? thanks!

if you could do that, that would be very annoying...

try facebook :-)