I also have this problem on a Dell L502X with nVidia 540M GT. I upgraded from Windows 7 to Windows 8 and in a couple of hours I started to feel my eyes getting tired. Then I looked at another computer here at my company and noticed the difference in text.
Since then I've tried all the suggested solutions with DPI, resolution, ClearType, switching cables, different graphic drivers and nothing worked.
I reinstalled Windows 7 but 1-2 months ago I tried W8 again hoping the problem was fixed... but its was the same.