Wise Way To Boot Into Windows With Bluetooth Keyboard?
If you can't access the boot menu at startup, you can always select which OS to reboot into from within OS X or Windows.

In OS X, just go to Startup Disk in System Preferences, and select your Windows partition, then restart.

In Windows, you need the Bootcamp tools installed (but they should have been already). You should see Startup Disk in Control Panel, which allows you to do the same thing.