Hollo ! How to write a PHP which executes at a given time ?

Dear all ,
 Hello ! I am going to write a suit of programme . The producers as follow

 producer 1.
 User A add an new appointment A ( "open Winamp at 2:30 on computer B")
through IE in his local computer A .

 producer 2.
 Computer A send the message to server A through http . The Servlet /
ASP.net which locates on server A receives the message and send message to
computer B (IP or domain name of computer B is given , named
"www.computerB.com") .

 producer 3.
 Computer B receives the message from server A through http at 1:01 . It is
expected to implement the new added appointment A ( "open Winamp at 2:30 on
computer B" ) at given time .

 I know what to implement the producer 1 and 2. However , I don't know how
to implement producer 3. Do you mind to tell me how to make a PHP programme
to execute a centian routine at a given time ? Thank you !
Jonathan