Compiler Errors

When I download any project onto Unity, it says: “All compiler errors have to be fixed before entering playmode” Why does it do this? When I updated to Unity 4 and upgraded my project, it started saying that for my project too. I tried changing the platform, but it didn’t help. I don’t know how to script, so I don’t know if it’s a script error or not, but all the scripts say they couldn’t load. I also clicked on the debug button on MonoDevelop, and it said the compiler crashed on every script.
Thanks for helping.

try this:
Go to your “Assets” folder, and delete every folder with “Standard Assets” in the name.
Then go to unity, go to the Assets menu → Import package → Image effects (and whatever other packages you want/need.

Will I need to re-download CarTutorial to import the assets again?