Ligaya Turmelle has posted a quick howto of the steps needed to get a dual install of Apache and IIS on a Windows machine, complete with PHP installed on both.
After installing PHP and Apache and making sure everything works perfectly - assume you already know how to do that. No one step is particularly hard but finding out what to do and how took time. Only took me 2-3 days to figure out this. Hopefully someone else won't have to wait so long.
The list is pretty straight-forward with only three main steps and a few sub-steps under each.