This week's book giveaway is in the OO, Patterns, UML and Refactoring forum. We're giving away four copies of Refactoring for Software Design Smells: Managing Technical Debt and have Girish Suryanarayana, Ganesh Samarthyam & Tushar Sharma on-line! See this thread for details.
I think you might have created two path environment variables. Try this on the console
set PATH=%PATH%;C:\Program Files\Java\jdk1.5.0_20\bin
Then try to run the javac command in the same console. Also I hope that your JDK is installed in the directory that you gave us i.e. C:\Program Files\Java\jdk1.5.0_20\ and there is a javac.exe in the directory C:\Program Files\Java\jdk1.5.0_20\bin...