Ahhh.... Beagle you have a tricky config if you wish to maintian security ...!!
I have to dash off now as the missus insists she has some tasks for me .. but I shall return...
Basically though....
When you set up an ad-hoc peer to peer network (which is what you are decribing) you use "Internet Protocol" for the comms between the two computers.
When they both have the potential for independent internet access this can create a posible security risk.
There is a way around it though but I'll need to double check.
I seem to recall you have to use one of the other networking protocols for the networking of the two PC's.
I'll dig out the info .... unless someone with greater knowledge than I can contribute in the meantime.
I'm sure that was the case with XP .. don;t know if Vista is any different!
Borrocks .... I've just read what you posted again .... I'm off up the wrong tree ... please do ignore me
