I recently took the plunge and "down"graded from Vista to XP. It was not as difficult as you often hear and I'm so happy with the improvement that I wish I'd done it a year ago instead of wasting the time trying to "adjust" to Vista!
But back to the point: Everything worked great after the new install but display quality was not up to scratch, especially text display. Then I also discovered that transparent png images on my site were displaying a colored background.
I finally discovered that IE6 does not support png transparency and that tipped me off. Of course, my older version of XP installed IE6. But when I upgraded to IE7 I also saw that my text and overall display quality improved immensely. I didn't know this before.
So if you are still using IE6, however unlikely, you'll get much better image quality with IE7 (or Firefox).
More importantly, many people out there are still using IE6 so if you use transparent png's on your site they will not be seeing them correctly.
Check this page for a way to fix this: Digital Web Magazine - Creative Use of PNG Transparency in Web Design