What does tuxracer do? Is it extremely slow?
If so, you probably have old OpenGL libraries floating around. Do an ldd $(which tuxracer) to find out which libGL it's linking against, and post what you get.
The Mesa demos may very well explicitly link against the MesaGL library, I don't know.
Which games are using software rendering? You may be able to run ldd on their executable too, and possibly find something.