WORKGROUP should be the same on all computers
Network Type should be Work (not Home)
-Network Discovery On -File & Printer Sharing On -Public Folder Sharing On -Password protected sharing On
Basically the above enables Win7 to work compatibly with WinXP (and hopefully Win2k) and still work with Win7.
Then, to share from Win7 use Sharing Wizard:-
Right Click folder, Share With, Specific People, select drop-down in dialogue box and select Everyone and click Add (if Everyone is not already in the list of permissions shown). Then click Share.
To stop a Win7 Share I recommend NOT using the Sharing Wizard. Instead, Right Click folder, select Properties, Sharing, Advanced Sharing and uncheck Share This Folder and hit OK.
Also make sure firewall isn't blocking access.