How to use Laptop as GPS

ive got a garmin nuvi not sure which model, but its pretty useless away from the city.
I see in a lot of 4x4 mags people using their laptops as gps...was just wondering how that works?
Obviously you download/buy maps and install them on the comp? Cause my mobile boradband connection again doesnt work that well out of the city.
Have never driven too far from sth east queensland so i dont need detaile desert maps ( yet) but a few times it would of come in handy to have more detailed info on where we were and how to get out easiest.

Whats the easiest/cheapest way for me to start using the laptop?

Oh also, i see people buy/download maps for their gps.... i never got a usb connection with that unusual? It was a cheapy but not too dear about $400 if i remember