If you don't already have a GPS receiver interfaced to your laptop then you'd probably be better off buying a specialist device such as the Tom Tom.
Personally, as I already had the laptop and the GPS unit, I bought "Navigator" software through
www.directions.ltd.uk. About £40 for the UK version.
The main advantage was that it was a cheap solution, the main disadvantages are that it's not optimised for operation with the GPS receiver, so it's always a "bit late" with the voice commands. (For example "Turn right in 100 yards" happens when you're about 3 car lengths away.) and it's VERY fiddly to force it to calculate the route you want (You have to ensure you set a waypoint on the correct carriageway of a motorway for instance - otherwise it's as likely to think you want to go the other way along that road - with very interesting results).
There's also laptop battery life to consider, for long journeys an in car PSU may be necessary.
Hope this helps