You can, however it is too big of a security risk. If you opened even restricted shares to the internet it is still very easy for someone with the time and skills to hack you. I would NOT recomend opening shares to the internet. Use a more secure method like maybe a FTP server.
They are on by default an can be accessed through the Internet if you are directly connected (no router or firewall). If you have a router simply forward ports 137-139 UDP and TCP to your computer. To access a share bring up the command console.
Type: net view <IP-address> to view the shares of an IP. Type net use x: \\<IP-address>\<share> to connect <share> to x:.
!!! WARNING !!!
This is a VERY hazardous way to share files. Windows shares are EASY to hack! You have been warned! Normally it is a good idea to BLOCK ports 137-139 UDP and TCP in your router and/or firewall.