If you do something to mess up Scratch beyond repair, just uninstall Scratch and reinstall it. It's not that difficult.
I always use the source code anyway, it's much easier than getting into the original program (and the code's a lot cleaner, too).
Offline
While yes, it's true, it's also true that it's ultra-useful to hack programs if you know what you're doing. I activated mesh by hacking Scratch. But I did mess up once so it worked so badly that even the X button didn't work- it said something like "button does not work" when you tried to close it so you needed to Ctrl+Alt+Del to close it.
Offline