I'm a computational chemist working with little computational power and dealing with increasingly demanding chemical systems. My work machines are part of a big network (mixed with windows and linux) and I was asking myself: is it possible to "borrow" computational power from machines around me (with permission from my department, of course)?
You can use HTCondor that is designed exactly to "steal" cpu cycles from remote machines. It may be a little difficult to setup but I think this may be the best approach.
You could install BOINC on those machines. When the computers become idle, the BOINC screensaver/client requests tasks from a server and computes them. See more information about it here. This is the software used by a number of projects such as SETI@Home. You can create your own project with BOINC and then put your desktop machines to work.