You're going to have to have some kind of "client" running on the other machine, machine B, inorder to control it. If pcAnywhere, its self" is your issue take a look at Damware or VNC. If it is ANY remote software then you have a little bit of a problem.
If you want to get REALLY REALLY involved you could do some script programming that checks to see if a certain file exists (we'll call it pcanywhereon.txt) and, if so, start the pcanywhere service. The script would have to be polling for the file constantly for this to work. To cause the service to turn on you would copy pcanywhereon.txt to machine B, wait for the script to to see it and start pcAny.
Obviously, this is a lot of friggin work. I gotta ask; why don't you want the service running on machine B all the time?
Jim