If the ISP rules and intent is to prevent file sharing you will not be able to break through. If you do somehow, when they catch you they will cancel your service. Ask the ISP if they allow what you are trying to do. They may say no or that it is an extra cost option. A friend has a firewalled ISP but they sell virtual private networking (VPN) connections as an option to get through the firewall.
That said if you can get through your options are:
1). Set up an FTP file share program. I like bulletproof G6. One of you hosts and the other can access using an ftp client like cuteftp or just by putting
ftp://his.isp.here.xxx into the IE address window. Bullet and Cute are pay for sofwtare there are freeware alternatives. Port 21 needs to be opened in the firewall to allow FTP.
2). Make your own P2P file sharing system. There is software out there for build your own P2P. Do a google search.
3). If you just want to swap files, not actually display a directory you can use MSN IM or ICQ.
Also, make sure you both have the windows firewall turned off when testing to rule that out as a problem. And if you have other software firewalls or hardware routers at either site make sure that port 21 is forwarded to allow FTP.
AND AS ALWAYS:
Make sure you have an up to date antivirus and a firewall up. Sadly the net gets uglier every day.