Mac Pro 3.33 here, This is a problem I had on my Macbook Pro before, which mysteriously resolved itself. From the Windows Boot Camp control panel, my choice of operating system won't "stick". Even if I try to switch to OS X, it'll still boot up Windows again. I have to hold the option key and switch at startup to get back to OS X. The OS X startup disk panel works normally. Tried reinstalling the Boot Camp software and resetting the PRAM. No dice!