Do what Mordor2112 and AMwuhihi say. This tutorial helped me
iClarified - iPhone - How to Downgrade Your iPhone 3GS, 3G Baseband for Unlock (Windows)
but with the iPhone2,1_6.0_10A403_Restore not the one they say there.
(and insist because redsn0w didn't work all the time, had to try a...