I am having this problem after updating windows where all photos in lightroom appear underexposed. When viewed from windows explorer OR when exported from lightroom they appear correctly exposed. I already tried to re-install both lightroom and photoshop but the problem persists.
Windows updates not infrequently overwrite your display's color profile with one that's corrupt or incompatible with LR. To see if that's the problem, see this article: http://www.lightroomqueen.com/articles-page/how-do-i-change-my-monitor-profile-to-check-wh ether-its-corrupted/