There is a dds file called (iirc) window_alpha.dds. Change the opacity of this will change the tint of the windows. RSC has some posts on the subject...
Look at post #4 in this thread. Thats the file you edit.
To edit it, you need to download the (free) plugin from nVidia, and follow the instructions to use it in Paint Shop Pro or Photoshop.
To find the stuff go to www.google.com (also try booble, but only cos it's got a funny name). Then type in "nvidia dds plugin". The download is sure to be near the top of the results.
Easy wasn't it! Bet you wish you'd thought of doing that on your own.