Just bought BiA Hells Highway. My specs are as follows:
Q6600 @ 3.4Ghz
4Gb Ram
9800GTX 512Mb
Windows Vista 64bit

I put the resolution at 1280X1024 in windowed mode and I get over 100 FPS. The moment I put it to fullscreen I get a max of 15 FPS. It doesn't matter what the graphics settings are or what resolution it is. Vsync is off as well.

Is there something stupid that I'm missing as from what I've seen my rig should be able to max it out at 1600X1200.

Update your video driver and stop overlocking.