Well, you can't really "access" the processes running ona remote machine, but you can get a list of them (and this is probably restricted with permissions).
Process processList = Process.GetProcesses("machine name");
Using a "." for the machine name will get a list of processes on the machine the code is running on.
To find all machines on a given LAN, look at the nMap utility. It might have a commandline version that produces output you could parse.