You could run them all as a cluster to encode video, for example check out the dvd::rip
cluster mode, but the real world performance will probably be significantly less than what you are expecting.
People tend to assume that a cluster is capable of incredible feats of processing, but this is really only the case in select applications and hardware configurations. Cobbling together a bunch of outdated systems (mismatched, at that), combined with the physical and software overhead entailed, means you will have a big complicated system that almost certainly won't be any faster than even the cheapest quad-core CPU on the market.
However the bigger problem is, as you said, all of those machines are already doing something in your network. You would need to have them all running Linux for this to work, which sounds like it is going to be a problem. Unless you had a setup to PXE boot them into a minimal system, but even then the machines won't be available for their intended uses (hope you don't want to watch anything on the HTPC while encoding videos).