What revision P5B do you have, and what voltage are you running to your memory?
I used to have a 1.03G P5B, and I had to have max vdimm to boot into windows on my 8x400 (3.2GHz) overclock. I then ended up getting a replacement 1.04G P5B, and the system would not boot into windows unless I had my vdimm set to 1.9v. Go figure...

-Ryan