Are you running Windows as a BootCamp install, or inside a virtual machine, like VMWare Fusion or Parallels?
I don't recall if Bootcamp had a virtualization layer, or if it was a "dual boot" where Windows had direct access to all the hardware.
Given all data so far, and assuming it's a Bootcamp "direct access" Windows install, then I'd take a serious look at the ATI 600 USB2 card:
http://www.amazon.com/gp/product/B00...SIN=B00138EOH8
Yes, it's labeled a "TV tuner" card.
Yes, it only shows a coaxial "screw in" connection in the photos.
Ignore that.
The card is a fully functional capture card, and it comes with a break-out bundle for s-video or composite input (+audio RCA), connected to a slot on the stick. It also includes a USB extension, in case you can't put the stick directly into the laptop.
It captures lossless HuffYUV AVI beautifully in VirtualDub, and it records MPEG-2 nicely in ATI Catalyst.
I still prefer the older ATI AIW AGP Radeon internal desktop cards, but this makes for a good second choice. Or for a laptop, the ONLY choice. I take this one on the road, just in case I need to capture on-location.
That AMD system you have would work perfectly for an ATI All In Wonder Radeon AGP card, just FYI. The two dedicated ATI capture boxes on this desk I'm sitting at are ~3Ghz systems, one an Intel with 1GB RAM, the other an AMD with 1.5GB RAM. Those computers are not slow/old for standard definition video work, not at all!