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Assuming the system is a Mac with Windows running on a Boot Camp partition, did you install the Boot Camp drivers in Windows 8? If so, you can change the default startup disk to Mac in the Boot Camp Control Panel.  Alternatively, you can hold the option key at startup and select the Mac volume, then log in and change the default startup disk to Mac in System Preferences.

If Windows is installed to boot natively from EFI, you may want to look up a thread on the MacRumors site. Search on "booting natively via EFI".