The short answer is, "no, not really". Not all that many common applications are designed to take full advantage of the parallelism of separate processors within the same computer, let alone across multiple computers.
Exceptions to this are the sort of parallel computing that was first popularised by seti@home, which takes a large problem of a type amenable to splitting into a large number of small workunits, and distributing them across a large number of separate computers. But even then, the separate computers are not really collaborating on the same task, just working on one of many identical tasks. Since seti@home, the WCG and other compendia of large distributed computing projects have made available many arguably more useful ways of using spare CPU cycles.
None of these make lots of little computers look like a big powerful Windows machine by parallelising the concurrent operating systems.
For all practical purposes, unless you want the large carbon footprint that arises from leaving many old computers on all the time, you're on a hiding to nothing with this idea.
If you're really keen, there are some home-grown applications you could do as a not-insigificant type of project. Here's a related question...
I stand by my more general "don't bother" position unless you want a noisy, power-hungry, specialist project to take on just for the fun and learning of it, though.