Network options (open folder) over Internet?

I want to open a folder of my PC to some friends of mine, as i can do it in a Network but now over internet. We have dynamic IP adresses and we’ve got different systems (Win2000, Win98Se) I don’t know how to find/adress him. To find the aktual IP Adress isn’t the problem … but how to use it? Someone an idea? Or a good software tool for me?