Hi xbxhkz and Welcome to the forum!
Since you've made sure that your Windows XP and Windows 7 machine are in the same Workgroup, you will need to install the Link Layer Topology Discovery (LLTD) Responder on your XP machine that can be downloaded here
. This will enable your XP machine to communicate with your Windows 7 computer. You also want to visit "Network Sharing Center" then select "Chance advanced network setting" and select the ones appropriate for your setup. To help you diagnose your network sharing issue, make sure that you have at least one folder shared both on your Windows XP and Windows 7 machine.
Please let us know how this turns out