First, install the latest Point Release. I believe it's up to 1.29 now.
Next, start the game, and go into the console by pressing the key to the left of 1.
Type the following:
/timedemo 1
/demo four.dm_66
The demo should start playing. If not, you probably did something wrong. With my settings and hardware (Athlon 1.1 / Geforce 2 Pro), I get 138 fps on that demo.
~bex0rs