Hi I know that it is possible to replace the default navigaton program on my mio C510 by replacing the files within the "MioMap" directory and renaming the tomtom.exe to the mio exe name.

My question is how would I go about doing this without fully unlocking WinCE? As at the moment i have access to WinCE on the deivce but do not know how to transfer files to it. So basically my aunt & uncle will be using the device all I want to do is when they click on the navigation icon it loads up tomtom instead of taking them to the WinCE desktop.

maybe you could start by launching a file manager (like file manager CE) to start moving files around from the SD to local flash or viceversa.
Or even using the softbutton application to launch some other wince applications.