I think it is your viewer. The easiest way to make sure, however, is to open an image in your web browser.

With XP® this is done by right clicking on a JPG file and pointing to open it using Internet Explorer...

...then compare that with what you see in Photoshop.

If it is the fault of the new image viewer in Vista®, I would scream at Microsoft® on a continual basis until you think each of their code monkeys have sufficiently worked up a bleeding ulcer © for failing to learn the lessons of Microsoft's®  history from Windows 95®­ until Windows XP®.

If history is any indicator, you can expect it will be another five or six years until Microsoft® develops another stable system.

Maybe, finally, someone else will develop a third OS to deliver the world from this two company tyranny of Apple's OS-X®© and Microsoft's® stolen IBM® code?

Yank My Doodle, It's a Dandy!