I just upgraded to Windows 10 and found Lightroom exported my photo much darker than it appeared inside Lightroom. I first thought this was a problem with color spaces (sRGB vs Adobe RGB, etc.). It turned out Windows 10 had recognized my monitor and installed a Color Profile for more optimal image. But I would rather see what my photos will look like exported while I am working on them. Once I removed the color profile, Lightroom’s too light images went back to normal. In my research, I read this is because Lightroom is color managed, but internet browsers and some image viewers are not.
In Windows 10, this setting is found in Color Management on the All Profiles tab. Then scroll down to ICC Profiles, select the unwanted Display profile, then hit Remove. I am sure it is possible, but I have not investigated how to get Windows to reinstall the profile if I changed my mind, so try this at your own risk.