Try clearing out your sessions cache (it might either be corrupted or there is something timing out during startup). If you have Xubuntu 13.04 or later, you'll find the "Clear saved sessions" button in Settings Manager->Session and Startup->Sessions tab.
If you have an earlier version, you'll have to manually clear it before you log in. At the login screen:
- press Ctrl+Alt+F1 to go to the first console.
- log in to the text console
- run the following commands:
- go back to the gui console by pressing Ctrl+Alt+F7
rm -rf sessions
- try logging in to see if there is a difference.
If you still have problems, ~/.xsession-errors or ~/.xsession-errors.old would be the log files that might provide more information.