If you keep swapping which USB port you are using the Roland in, Windows 7 will have to re-install.
Always plug the device into the same port or leave it plugged in and power it down and it should not re-install.
If this does not work then your Windows 7 has an issue in that it is not saving the device 'settings' - if this is so then have a look on the Roland site to see if others have had this issue and if a fix is available.