i had the same need - i was asked to do 2 things - i had to do them with limited resources and right then and there. there are numerous alternatives out there, in addition to customizing your own application, but with limited programming experience and funding, i had to find immediate solutions.
1) how can we view a real-time map of an entire fleet of vehicles? the answer for me was GPSGate - i had it done in 1 day for free on a trial version, and very cheap thereafter the trial.
2) how do we enable our fleet of 250 to have our entire GIS database on a map on their laptops with real-time display of each of their locations? the answer for me was TerraGo Tech's GeoPDF - I had it done in 1 day for free with a trial version and the free GeoPDF toolbar on every laptop, and very cheap thereafter the trial (i basically can have a huge mobile fleet of thousands with real-time GPS and GIS with 1 Map2PDF Bundle that cost around 2K). This blows away the alternatives like ArcPap, ArcGIS Server Mobile, programming a stand-alone ArcReader ArcObjects application, single-use ArcGIS or ArcView licenses for every laptop - absolutely nothing competes $ wise, unless you are an expert programmer and have time, which i am not and didn't have.
A great free tool you may like for your purposes is DNRGarmin.
Try GPSGate, GeoPDFs, and DNRGarmin out and let me know what you come up with because I am always looking for other cheap or free alternatives to real-time GPS with our customized GIS data.
Although, the simplest is if you have a single-use ArcGIS license you can use the GPS tool in version 9.2, or you can use DNRGarmin with ArcView 3.x, and again, that is if you have single-use licenses on you mobile laptops/units. I'm not really sure exactly what you need to do, but hopefully all the options I mention above may in some way get you started at least.