Networks, homegroups Windows 7 (and XP)

I've failed to find the answer on exisitng threads but if I have missed it, apologies. I am trying to link mjy laptop (W7) with the new PC attached to the TV acting as a media server (W7) with two Pure Sensia iintenet radios and wife's laptop running XP. I have a wireless internet running though the house.

The internet radios see the internet and see the media server PC and play happily.
My laptop has a netword pasword. The PC media server has another password.
My laptop and media server see each other but demand a user name (what?) and a password (which one?) so I cannot see the shared files. The XP laptop sees nothing and is invisible.

Any kind , wise people out there??

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