I remember installing a product a few years ago, called WindowBlinds.
It gave a total makeover of the look and feel inside of Windows. Was great, only problem was that it took up too much in the way of resources, at the time I think I was running an AMD 450mhz with 64mb RAM.
Well, now that I have a better spec I thought I would download it again, and what a treat it is. Running on this system it goes great, and looks great.
There are hundreds, if not thousands of downloadable skins from the web.
My favourite so far is one of the 7 that come pre-bundled with it, that gives it the look of a Mac OS. Even replicates the button functions, ie. the far left button at the top of each window closes the program, just like in MacOS.