Do you want to share them inside the lan or to people outside the lan?

Although Comcast doesn't advertise a static IP, as long as the MAC of the cable modem doesn't change you should have the same IP for many years. I'm on Comcast and before I changed my cable modem I had the same IP for 3 years. Had I done a mac change in the new modem I probably could have kept that IP for longer.

The best way to share a folder to the outside world is to setup a simple VPN connection to your XP box. In the XP connection wizard you can set things up to accept incoming connections from the outside world. You'd need to open up some ports on your router, but it'll be much more secure than most other options.

You could also consider a simple FTP server on the machine to do the same.