where, there could be a couple of different issues going on. in a workgroup there are 3 main things that need to be present. 1. all computers belong to the same workgroup [and not the default workgroup setup during installation. you have to join the computer to a new workgroup even if the name is workgroup]2. user accounts must be present on all computers which will shared folders or files within folders. Example: If Jim shares a folder on his compuer for John, Johns user account must be on Jims computer.3. Ya have to share something.
There is also a security policy "access this computer from the network" in the local machine security policies. sometimes this gets changed by people who think deleting users or groups from it secures their computer. Such changes often times causes a lot of problems. Here an article: http://support.microsoft.com/default.aspx?kbid=823659
In XP unlike W2K, there is local sharing, network sharing and configuring the Windows Firewall to allow the folder to be shared with others on the network. If you right click a folder and choose sharing the folder properties windows will open to the sharing tab. To allow others on the network to access the shared folder you have to check the box in Network sharing and security. Just dragging a folder to the Windows XP All Users shared folder allows only local machine sharing.