Windows Explorer Error Message

When I start the computer, a error message ran as follows: “Windows Explorer has encountered a problem and need to close. We are sorry for the inconveniece.” Then I clicked on “Send Error Report” and then “Close” on the “Error Reporting” window. This sequence kept occuring forever. I got into the Task Manager and end the process “Explorer.exe” the message did not come out anymore but the “Start” button and the Task Bar disappeared and I could not get into any application. Anyone has any idea what happens and how to fix that? I am using Windows XP Professional on a Dell Dimension 340, 512MB RAM, 40GB HD. Thanks in advance.