I think the problem happens because OSX corrects the frequency response, while Windows drivers
don't have any processing.
I measured 24" iMac response and created DSP code for Windows7 to apply the correction, and the software is available at the link below. The solution doesn't work on Windows XP or Vista however. http://yohng.com/software/imacsoundfix.html
I also noted, that the microphone is switched on by default, which creates unnecessary feedback in the sound.
Please check the web page for the instruction on how to switch the microphone feedback off.
The measurements were taken from my 24 inch iMac, so it's hard to say for sure, how this algorithm will perform on a computer, which acoustically is different from 24 inch aluminium iMac.