First: where, exactly, did you install JBossAS? Full path, please. (I hope that your step 3 did not mean what I think it does - I would not install JBossAS into the Eclipse directory.)
Second, when you configured the JBoss4.2 server entry in Eclipse, what were the settings that you used.
Third, after setting up the server, did you try running it without having any projects associated with it yet? If so, does it run?
Fourth, was there a stack trace provided with the null pointer exception? Could you post it. If none was shown to you, look in the workspace/.metadata/.log file - the full stack trace might be there.