Hm, that is quite odd...
Have you also applied the latest Patch for the Demo ?
The Patches for UT2003 and its Demo resolved quite a number of Problems with 3dfx Cards and this Game.
Normally, it should run.
(Check my early experiences with the
unpatched Demo here :
http://www.falconfly-central.de/cgi-bin/yabb/YaBB.pl?board=games;action=display;... )
Things should run much better than that now.
Also, have a look at the 3rd Party Drivers. Some of the very latest Drivers were tested quite well with UT2003...
In case the Demo does not set the required settings in the UT2003.ini (to prevent or troubleshoot problems), you should have a look into it. Modifying the UT2003.ini (Direct3D Renderer Section) was always the Key to success in the Past.
PS.
I run it with an ATI Card, so no NVidia is needed anywhere with R8500, R9000(pro), R9500(pro) and 9700(pro) available to do the Job just fine
Nonetheless, the Voodoo3
can do it , although not at record breaking Image Quality or Performance