Low framerate

[luckymoney]

hi there
i have just download the demo.
found out that the fps is around 17-19,
it never go over 20 even in the main menu
tried the tweak guide, still no different
i can play UT2004 without problem
ihappens in demo only or in retail version as well?

my system
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CPU: AMD Athlon 64 X2 Dual Core 5000+,
Memory: 2G RAM
Display: 8600 GTS
OS: XP


thanks for help