Changing com ports

[bobbles]

I am running four rockminers with BFGminer on Windows 8. Initially they took com 4, com 5, com 6 and com 7. Everytime the machine is rebooted though the last two keep moving up ports (com 8, com 9 etc.) I can't move them back down because Windows tells me those com ports are in use. Ultimately I don't care which four com ports they use as long as they don't change. What can I do to stop this from happening?