I have found another way as but it depends how many users you have. I have removed the text box and added a drop down box.
<html>
<head>
<title>Untitled Document</title>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
</head>
<body bgcolor="#000000" text="#FFFFFF">
<%@ LANGUAGE = VBScript %>
<%
' Sending a message using NET SEND on a Windows NT network
' (C)opyright 2001 - Robin Paardekam
' Nice script to embed on your intranet. Send a message to any user
' on a network using your browser (!)
' More scripts like this can be found at
'Checking for all necessary variables.
Msg = TRIM(Request.Form("Msg"

)
NamUsr = TRIM(Request.Form("NamUsr"

)
Computer = Request.ServerVariables("REMOTE_ADDR"

if computer= "10.0.0.72" then user= "User1"
if computer= "10.0.0.13" then user = "User2"
if computer ="10.0.0.154" then user = "user3"
if Msg <> "" AND NamUsr <> "" THEN
'Sending the message
Response.Write "<P>Sending message to "
Response.Write "<B>" & NamUsr & "</B>: "
Response.Write Msg & "</P>"
Set server_shell = Server.CreateObject("wscript.shell"
server_shell.Run "%comspec% /c net send " & NamUsr & " [FROM:" & user & "] " & Msg
ELSE
'Displaying the form %>
<p> </p>
<p align="center"><font face="Arial, Helvetica, sans-serif" size="7"><b>SEND MESSAGE</b></font></p>
<p align="center"> </p>
<p align="center"><img src="images/email_computer-ani.gif" width="107" height="35"></p>
<p> </p>
<FORM NAME="Vorm" METHOD="Post">
<div align="center">
<p><font face="Arial, Helvetica, sans-serif" size="5">To:</font>
'ADD THIS SECTION INSTEAD OF THE TEXT BOX
<select name="NamUsr">
<option selected>USER1</option>
<option>USER2</option>
<option>USER3</option>
</select>
</p>
<p><BR>
<font size="5" face="Arial, Helvetica, sans-serif">Message: </font>
<input type="text" name="Msg" size="60" value="">
</p>
<p><BR>
<INPUT TYPE="submit" VALUE="Send Message">
</p>
</div>
</FORM>
<% END IF
%>
</body>
</html>
I have not tried it but I would think you could put in the persons name and do an if statement to change it to the machine name to send.