"net send" is a command using a background service called "messenger". This service has been removed from Windows 7. ie You cannot use 'net send' on Vista nor Win7 / Win8.
Pity though , I loved using it.
There is alternatives, but that requires you to download and install software on each pc you want to use, this software runs as background services, and i would advise one to be very very very very careful of using these kind of software as they can potentially cause seriously damage one's system or impair the systems securities.
winsent innocenti / winsent messenger
****This command is risky because of what is stated above***