Not using a GPS before but, i would have to say Paying for a service tends to mean your getting the Best. Nav4All might be free but it could also be Wrong, or missing stuff in its directory.
If the company is advertising and they have a Large amount of customers the chance of getting a virus are slim to none, but there is always a chance
You can also look on
www.google.com for a forum to do with GPS Nav tools, and they might be able to help you.
Also, Posting this in the Lounge would be the best place if theres no Other Hardware Discussion Board.