Unfortunately, Jacks PC and graphics card are very old. His game not launching , is a Direct 3D error, due to the card not having the ability to launch the game. It won't matter whether it's Demo2 or the full game itself. My Dell 3000 had the same problem and it was a newer PC than Jacks, fortunately mine had a PCI expansion slot. Jacks is so old he doesn't even have a means to add a new card. His Pentium 4 and 1.2 G of RAM are fine, I am using the same processor and 1.0 G of RAM, difference is the Nvidia 512 mb card.
Sorry Jack, a new computer will be the way to go. Don't know if your in the USA, if you are, Walmart sell a base E-Machines with no Monitor for $290. I looked at this PC for my son, It has plenty of RAM and Processor speed etc.. It has an onboard Nvidia card (which will probably work), and the ability to add one if that doesn't work.
Let me know if I can be of any more help, you could send a private message if you wish.