i've got a
P3 450,
256 mb ram
tnt 2. i've got 1gb space after installing. The disk is defragmented every 15 days

The games runs jerky in 640*480, with full detail.

My friends got a
p2 400
64 mb ram
tnt2

and the games runs superbly.

What do i do?

Try closing all apps before playing. There might be some apps hidden in the tray system like Quick Time, Real Player, some voice or audio apps, U can close them with Ctrl+Alt+Del and close all apps that you know you don´t need.

Also download all most recent drivers for your TNT and DirectX 8.1.