Okay, a few questions:
Are you logging in from the command line (using startx, startxfce4), or a display manager (KDM, XDM, GDM)?
If you're using a display manager, have you tried choosing a different session, such as TWM (which will appear black but in which you can use the left mouse button at least to open an xterm)?
If you're using the command line, have you tried startx, startxfce4, etc.?
The answers to these questions might help narrow down the problem.