Thanks gents - I have cracked most of the problems with the mixed network (lots of bits shooting in all directions quite happily, touch wood!) and yes, Vista and XP really don't seem to like each other much initially - its just this problem with the machine where I can't access mapping tool (I have some notes about how to do it at registry level but would rather not play there!), I believe there is somewhere to turn mapping capability on/off on that machine but just can't find it.
GG - thanks for that, but i alraedy have service packs installed so have the advance network pack by default (just can't turn it on apparently!)