This method may not work for everyone. If it doesn’t I would try Apple’s recommended solution of a backup, reinstall (this step may not be necessary, a reformat might be good enough), and restore.
The easy method:
- Restart OS X
- Hold the keys cmd+s for single user mode.
- Type “fsck -fy” - this will do some filesystem cleanup and should be safe (unless you have hard drive or filesystem corruption - if you’re worried about this run disk utility repair first)
- Type “exit” to continue booting - then try boot camp again.
This worked for me, YMMV (your mileage may vary).