How to use the NET SEND command from Windows 7 to Windows XP?

In Service pack 2 the Messenger service got disabled by default.

To enable it type services.msc in Run and find the messenger service and enable it.
You'll need to enable it on both computers

Sysinternals PSTools may be of some use for pranks such as these.

I once combined PSExec with a burp prank exe to very great effect, right up until the branch managing partner (law firm!) walked round the corner at a most inorportune moment! O_o