Ok, I'm not sure what's going on here but perhaps someone can help.
I used Windows 7 at home now, before I had Vista. At work I use XP.
I've noticed the last photo I uploaded to (and it's not great) appears much darker on my PC at home. This seems to be related to the OS as a friend running Vista also has it appear darker whereas a mate running XP sees it as intended.
For reference the image is of the moon, framed by some trees. If the photo appears dark, you can only see the moon and clouds.
In Lightroom (2.4) I see the image as intended but in both Windows explorer and Firefox under Win7 it comes out dark.
However at work where there is nothing special going on (except that the OS is XP) it works.