I am strugling with OpenCPN
also. I have a new computer with windows 7 also, I just bought the Garmin GPS
18x USB and downloaded Spanner 21 a couple of days ago and seems to work for me. I must add that I think it is working because I do get the right coordinates (for my house, where I am trying this)) at the bottom of the screen
under ship coordinates. Now I do not know what I did to tell you the truth.
My problem is that I can not get the charts
that I downloaded to show up on the black screen
with the outline in blue of the North American Continent
However under "available Chart Directory" which shows everything in the C: Drive I can see "Garmin--Spanner set up.exe"
Where you able to get the charts
to show on your screen?
I am very challenged when it comes to electronics
Hope this help you somehow. :-)