Need help with Rollercoaster 3 demo

[Lokizen]

When I try to play the demo for Roller Coaster 3 the screen goes all sorts of black/grey zigzag and I know it's the fault of my video card.
I've updated the drivers for it and re-installed directx 9.0c, no avail.
My vid card is a PNY Verto, GeForce 8600 GT running on a 2.8 mhz pent 4 computer. Any ideas? Perhaps some settings I could tweak?

 

[DEATH eVADER]

What O/S are you running.

I also thought I'd mention that RCIII becomes a lag-fest after about 15 minutes on a Vista setup (I learnt this when my nephew built a park from scratch)

 

[Lokizen]

XP Home.

 

[Lokizen]

Just had a thought... Would an update of my video codecs help? One of the suggested fixes is also to update the sound card drivers. Anyways, still surfing for some ideas.