If you can overclock from windows but not from bios. Then one of two things are happening.
A) You are not properly overclocking in bios. And the software is making adjustments other than just upping the FSB.
B) You computer is not stable which is causing it not to boot.
Have you ran stability tests after overclocking in windows?
From what I have read on various forums, there is an issue a fair number of people are getting where the Gigabyte DS3P bios won't allow overclocking to be enabled when using an Nvidia card. The only solution that has been found is to 1:- Use an Ati Card or 2:- Boot from a PCI card, enable overclocking then shut down and put your pci-e graphics card back in.
In windows, my computer is stable at 3ghz and I have tested it as 100% stable... and have in fact clocked it higher as of last night.
I can't even enable the overclocking options in the bios at the moment.