The problem is that your ISP probably is doing some kind of filtering to disable this kind of stuff. Not nessisarily beacouse they don't want to you to do that, more so beacouse they don't want people to acidentaly share there entrie harddrive with the whole neiborhood.
To do what you guys want, the best way would probably be to set up a VPN "virtual private networking" connection between your two computers. In windows 2000 it's pretty easy. What the VPN connection will do is create this virutal tunnel between your two computers and it will pretend like there on the same lan. VPN can also be very secure to keep the hax0rs out.
It's been a bit since we did this in class so I might not be exact here. You may be able to find something in Help (which is acutaly pretty good in Windows 2000) or on microsofts web site.
Open "Routing and Remote Access" in admin tools on the server. Right click on the server and go to "Configure and Enable Routing and Remote Access". Suprise, a wizard! Click next. Select "Virtual private network (VPN) server". The only protocol you need is TCP/IP. Select the adapter that talks with the internet. If you have DHCP already installed on the server just select automatically. If you don't (this is what I will walk through, just in case you don't) selec the second option. Create a new range. You don't need a RADIUS. The server will now start the routing and remote access service on the server.
On the client (I hope your running 2000 as well, I'm not sure how to do this on 98) right click on My Network Places and go to properties. Click on make new connection. Click on next, select Create a Virtual Private Network (VPN) connection or 'tunnel' through the Internet. Enter your friends internet IP address (if he has dynamic, your going to have to reconfigure this step every time he gets a diffrent IP address). Next option is up to you. Click finish and your done.
Make sure you have a username and password on the remote computer and that account has dialin or VPN access. On the client just type in the username and password.
Do some ping tests, check out tracert. You should be setup and fine! If you have any other questions feel free to email me at clark_l*nospam*@efn.org. Just take the *nospam* of the address to get my real email
. Good luck!