"Could not find the main class:processing.app.Base. Program will exit" Error.

Momac99:
I've got an UNO R3 and I'm using windows 7

Excellent. Now, what doesn't work? I mean, other people with an Uno use Windows 7. So, it follows that it works for you. If it doesn't, you should share the details. All of the mind-readers are on vacation this week.