If you experience an unbootable windows partition, short of reinstalling windows, some options are:
While starting the computer up, hold down the 'F1' key to get to the windows boot screen, allowing for a memory scan, and 'F8' to select the various "safe modes" to boot into.
The 'F8' menu also contains the option for the 'Last Known Good Configuration' in-which the machine (hopefully) was recently booted from.
Of course, you can also hold down the 'Option' key while booting, to select another operating system partition to boot from.
