distributed computing is not really goint to help, it will just allow you to run two computers that can talk to each other which is not what you are after.
What you can do, and i think someone mentioned it above is build a cluster but for general computing it is not going to be much help to you. A cluster is just multiple computers joined together and allows for load balancing (i.e. if one of the cluster is busy with a process new processes will get passed to the other computer) - This is used with webservers and the like.
But really the answer to your question is no, you cannot really, especially if you want to run them as normal computers. Your best bet is take the best parts of each PC and put them together to build the best PC out of what you have.