I've been thinking about this problem again and I think that I am making it much more complex than it needs to be.

Each PC that is available to perform an operation with a flat file could dip in at intervals into a single folder (on a network) and then remove the longest present flat file (request file) to the PC in question, so preventing other PCs using it. The process of grabbing only takes place when no other flat file is being processed on that machine. The file taken would be the one that has been in the holding folder for the longest period. The time intervals that each PC attempts to grab a file could be determined by CPU activity, and this would determine the probability of each machine taking on the task. The time intervals in which the holding folder is checked could be set so that it is impossible for two of the PCs to attempt to take a file at the same time, although I have no idea how I would work that out. Can anyone help me with that? 2 or more PCs taking a file at the same time is the only problem I can see with this.