Since my last post I've not had a lot of time to sort this as it was my weekend on, and I've been busy.
I Googled 'xp desktop can't see vista laptop' and got reams of links back! I think it might be the most wriiten about subject on the planet. The Ms forums have shed loads of stuff on them.
I found some stuff about firewall settings (both machines have identical firewalls with default settings), but I have found some things that look promising. Just need some time to try them, earliest will be tomorrow night as I'll not be home til about 11 tonight - another 18 hour day
There was also an interesting thread in a computer forum about fooling Vista into thinking the printer (attached to the XP desktop) is really a local printer. Something else to try if I get bored with the network issue!
I'll keep this thread updated.