This week's book giveaway is in the OCPJP forum. We're giving away four copies of OCA/OCP Java SE 7 Programmer I & II Study Guide and have Kathy Sierra & Bert Bates on-line! See this thread for details.
I suspect that either your Eclipse workspace or installation is corrupt. Try this: create a new workspace and create a simple project with a main() method in it and see if that works. If it does, there is most likely something corrupt in your old workspace - move oll of your projects to the new workspace. If you still have the same problem in the new workspace, then most likely the Eclipse installation is corrupt, download and reinstall Eclipse.