It is not easy to suggest a mapping product w/out knowing your geographical location, but here are some quick possible solutions. Streets & Trips (USA, Canada, Mexico) and AutoRoute (Europe) are decent desktop applications, and they work with GPS, too. You could plan your trip the night before, and then just save the small file to either a memory stick, or let it be downloaded by your CarPC via the net or via your home network, or email it to yourself.
The question is if you like to use the above software on your CarPC. Some people like it, some don't. The people who don't like it, don't like it because it still is not as nice on a small screen.