You do NOT want to enable NetBIOS over your internet connection!!! Thats the most common method that script kiddies use to hack you and/or use your machine as a drop site and/or attack launch point. It's quite simple to do. All they have to do is scan a range of IP addresses for port 139 and presto...they have you. The also cannot typically be tracked down as they perform these vulnerability scans for other machines they have compromised. Your typical script kiddie has access to dozens of machiens and a serious hacker has full access to hundreds (if not thousands) of machines and chain the connections across different backbones so as to limit the ISP's ability to trace the packets and scan the logs.
The best way to transfer occasional files to/from each other would be to use some sort of IM application - like MSN Messenger, ICQ, etc.
If you are looking to frequently transfer files back and forth, you'll want to look at setting up an FTP server. Alternatively, if you are looking for Windows networking functionality, you should setup a VPN.
There are many other methods to share files, but whatever you do, do not use NetBIOS over the net without encapsulating it within some form of encrypted protocol (ala VPN) and get yourself a firewall - regardless of if you start transferring files.