Mac – Arbitrary Key Remapping on a Mac

Try DoubleCommand.

DoubleCommand is software for Mac OS X (a kernel extension) that lets you remap keys, in other words change the way your keyboard works. Often used to make a PC keyboard more comfortable with a Mac, swapping the Alt (Option) and Windows (Command or Apple) keys, since they are in swapped positions on Mac and PC keyboards. DoubleCommand is a quick and easy way to fix this if you use a PC keyboard with your Mac.

Try KeyRemap4MacBook. It seems to offer a lot of options. Maybe it will fix your problem as well. Here's what it says about it:

This is a very powerful keyboard remapper for Mac OS X. In addition to simple key remapping, it has special remapping modes like Emacs-mode, SandS-mode (Space and Shift).