This depends on what you mean by "user activity", on Linux you can get user logins with for example, Sys::Utmp or via WMI on windows, process information you can get via Win32::Process::Info or Proc::ProcessTable on Unix for instance. It is unlikely however that you will find something that will provide this information for both, I would guess that your best bet would be to create a "service" on each different machine that uses the appropriate method for the OS and expose that over the network so that the information can be captured by a single program elsewhere.

Of course both platforms support SNMP which can possibly be configured to provide the information that you need, and can be accessed over the network.

/J\