This week's book giveaway is in the OCAJP forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide 1Z0-808 and have Jeanne Boyarsky & Scott Selikoff on-line! See this thread for details.
Most likely in your experimentation you created your own class named "Thread" or interface named "Runnable", and either the *.java or *.class files are still around on that machine. Use the Windows "Find File" feature to find Thread.java, Thread.class, Runnable.java and/or Runnable.class, and delete any ones you find that you may have created yourself.