This happens to all p67 boards. To correct it, just manually set your BLCK to "100" in bios. That should fix the double boot.
Using the auto overclocking software will almost always set the BLCK to a number higher than 100. The advisable route is to manually OC your cpu in bios and ditch t he auto software (even uninstalling it totally).
A good place to start learning how to manually OC using an Asus board is here:http://www.clunk.org.uk/forums/overclocking/39184-p67-sandy-bridge-overclocking-guide-beginners.html
Hope that this helps you out.