I know there are other threads on this but I haven't read anything on NaviPC as yet.
http://www.navipc.com/
It uses the Sdal data format which is used by VDO, Fiat etc so Data CD's are fairly easy to come by also if bought from NaviPC they offer a 33% discount on the yearly update... which is nice
I just had a new car dropped in my lap.. which is also nice

Fiat Stylo which uses Nav+ software from Nav Tech. If any one has access to Navi PC or uses it I would be interested in their comments.
It has diection icons, Voice directions and map vies to name a few.
Looks great for Car PC projects in the UK and I can vouch for its effectiveness in the Fiat.