if you have an AGP slot, just pick up a cheap Video card with an S-Video out. I bought an ATI Radeon 9000 for my media PC that has S-Video out.
However, if you don't have an AGP slot, you'll have to go PCI, and that can get expensive. The Cheapest on Newegg is $1 shy of $70. YOu can get a new motherboard for that much money. One that has AGP or PCI-E on it.
I'm sure there are DVI/VGA transcoders out there that will convert your signal to S-Video, but they will be quite expensive.