The clean install will need the patches to work now (the 1.2 patch, the CPU patch and either the DirectX or OpenGL patch).
The demo will likely already have these. Make sure Cool 'n' Quiet (AMD CPU) or SpeedStep (Intel CPU) is disabled in the BIOS and I think I remember GEM+ having a workaround for it (though I've never tried it).
For the graphics glitch try the other renderer - if you use Direct3D try OpenGL and visa-versa. There is also a 3rd-party update of the Direct3D rasteriser which can be found
here.