There can be a lot of things that interfere, but these steps are pretty much required. (There should be an extensive tutorial on this in the "Tutorials" section of this forum. But here is a start.)
1) All computers must connect to the same router, (either wireless or wired) and thence to the internet.
2) All computers must be assigned to the same "Workgroup", and have different computer names. In XP Right click MyComputer->Properties then Name tab. In Windows 7, Rt-Click Computer->Properties-Change Settings (On the right, by "Computer Name".
3) To allow sitting at computer "A" and view/edit files on computer"B", first go to "B", and find the folder you want to share, then Rt-Click->Properties->Sharing. Go back to "A", with fingers carefully crossed, in Windows Explorer, Under "Desktop" select "Network", then on the menu, click View->Refresh.