Old Sat Nav?

I wouldn't worry. You'll typically find that the navigation CD is made by one of a couple of companies, such as NavTech -

Is it? How would one load them? Laptop plugged into the car somehow, or does the unit have it's own drive?

Usually it's combined with an audio CD unit - either in-dash or boot-mounted changer.

Well I didn't know that. Does the CD stay in the drive all the time, or is it just uploaded to RAM in the satnav?

CD stays in the changer.

The only exception I can think of to this are the new-ish generation of audio head units with combined GPS from Blaupunkt and Philips.

Stays in the unit all the time. The amount of data is significant and you'd need a lot of RAM to hold it all.

They all have a CD or DVD reader, either in the unit itself or remotely mounted in somewhere like the glove compartment, under the seat or in the boot.

My Stilo Abarth has a slot-loading CD in the main Sat-Nav head unit. A recent Nissan Primera T-Spec I drove had a DVD reader in the dashboard, just below the display.

If the Toyota Avensis figures in your list of cars then it's possible to change the CD that operates the sat-nav easily. The sat-nav unit itself is located in the boot and is a totally seperate unit to the CD player in the car. Toyota charge around £130 for an updated CD but I'm sure there are less legitimate ways of getting this....

Thanks for all the replies, I understand it better now.

I've got the Sony single DIN unit which came out about a year back. The Nav CD goes in the head unit and stays there.

If you don't have a changer, you can still use the head unit as a CD player even when using the navigation. It simply uploads the route details (and surrounding area) to its 8MB RAM and then you can swap the CD for an audio one. It will prompt if it needs the disc again.

Personally I couldn't be arsed with that palaver so got the changer to go with it.