Listing Processes Running on Remote Machines

[yoda]

Hello Hackers,
I'm developing a large scale distributed service and part of the
requirement is that I be able to monitor clients in a very granular
way.

To this end, I'd like to know if there is any way to get a list of all
the processes running on a remote client\machine.  I need to be able to
do this on demand. (i.e on user demand)

Please note that the clients run heterogeneous operating systems mainly
Linux and Windows2000\XP