Using Navigator on the country side!

[sigurdur]

I'm using TomTom Navigator with my Ipaq 2210.
Could anyone inform me if it is possible to use the TomTom Navigator outside cities? I.e. is it possible to use it while, for example walking on the country side, or in the mountains?
Is it only usable with preinstalled maps?
If so, can one use maps produced by third parties?
What format does the map have to have to be usable by the Navigator?

All these questions...
why? Because, I'm in Iceland where it comes in handy to use your GPS while driving in the mountains, or walking!

I used to have Palm V where I could use a GPS regardless of any specific maps. There, one could mark a start point, the Palm V drew a line from A to B etc.

Any hints, would be highly appreciated, thank you.