There are many reasons why windows can hang on startup. It can be a virus or a corrupt assembly (a corrupt .dll or .exe file) which is crucial to startup.
Under these conditions there might be a black screen appearing or the desktop may not load properly with incomplete task bar and start menu.
Under these conditions many people may not even be able to go into their system. One thing that you can do under these conditions is that, while booting, go to safe mode by pressing F8 repeatedly.
Then in safe mode, go to system restore and try restoring the system to a last known good configuration.