In my XP (via Parallels Desktop), the images are slight distorted and the quality appeared deteriorated.
I use the Direct Resize extension with ‘2’ for ratio option, so I am going to run a test and set to ‘0’ to see if it makes any differences, however, I checked Magento demo site, and is seeing the same thing.
Please see screenshots -left image is from Mac and right image is from the PC.
Safari vs IE 7
Opera vs Opera PC - the image from the Opera PC displays much better than the FF and IE
Firefox vs Firefox PC