Well the problem with Terraria is that it didn't install correctly. Try installing XNA again. Apparently there is an install executable in your Terraria install folder. Or maybe uninstall it and re-install it.
...but you say all your steam games don't load?
Did you do something recently to your computer? Someone had a similar problem that turned out to be his graphics driver had been corrupted.
Right-click anywhere on your desktop and choose screen resolution. From there click Advanced Settings and tell me what your adapter type is.