Explorer not starting

Thank you all that answered my last question. When i log onto my win2000 pc. Explorer opens up in a dos window then disappers. None of my icons load. My start toolbar is missing. I can run probrams using the run command through taskmanager. If i tryto open explorer through the command prompt. I get an error message. Program to big to fit in memory. If I browse through the lookin window MY Copmuter icon is the windows page icon. Any ideas help.