Ever since I got Windows 8.1 preview, Portal 2 seems to crash after 5 seconds when I launch it. I could launch it fine when I had Windows 8! Has anyone got it to work on Windows 8.1? If so how?
Various answers here and here indicate that uninstalling and reinstalling the game will fix the problem (none of them mentioned if they tried verifying the game cache first, which should theoretically do the same thing, but be much faster).
This post also suggests that you may need special graphics-card drivers for Windows 8.1, which you should be able to obtain through Windows Update, or (failing that) from the manufacturer's website.