Today one of my friends faced this problem and asked me what’s wrong it may be. This is a frequently asking question. And I know not only him but you also have faced this problem at least once in your asp.net life.
So here is the article from Microsoft which provides a list of articles that describes some of the most common problems you may encounter when you are debugging asp.net applications in visual studio 2003.
http://support.microsoft.com/kb/306172