Download a Linux distribution like ubuntu 10.04. or 10.10 not higher.
Transmission bit-torrent client will be pre installed. Better, install KTorrent from the software centre.
In virtualbox, set the Network adapter to bridged. This way your linux OS guest system can set-up its own connection.
Set-up a shared folder either through virtual-box and virtual additions or through samba shared folder.
In Linux, install gufw Firewall configuration which has pre sets for file sharing software.
Last, it's more secure than running p2p on your host/main system. https://www.virtualbox.org/wiki/Downloads
Regarding the Original request; Do you have a firewall router?
You might want to ask your isp and if you do how to configure it.