I also got this same problem from last day. i tried 2-3 time to uninstall/install android sdk from my computer. I install new eclipse, new workspace, eclipse update. every this i did from last day. but i could not run a HELLO WORLD Project.
Where ever i run my project;
Run as > Android Project, A dialog is populated
("YOUR PROJECT CONTAINS SOME ERROR(S), PLEASE FIX THEM BEFORE RUNNING YOUR APPLICATION").
i am working on android from last 1 year. but i never faced this type of problem.
If all else fails, try deleting ~/.android/debug.keystore. Sometimes your debug key expires, and you need to sync up with the SDK.