On the Windows 7 machine, click on <b>Start </b>and right click <b>Computer</b>, then <b>Properties </b>(or right click on the <b>My Computer</b> icon on the <b>Desktop </b>and click <b>Properties</b>). A window with system information will appear.
At the bottom of the window you'll see Computer name, domain, and workgroup settings. Click on <b>Change Settings</b>.
Under the <b>Computer Name</b> tab, click the <b>Change</b> button. In the next window, select <b>Workgroup</b>.
You should now see your XP/Vista machines shown on the network, and shared files/folders will be accessible, but you may need the network password if one has been allocated. The Windows 7 machine will appear on the rest of the network, but when you try to access any shared files/folders, you'll probably be asked for a password. That's because, by default, Windows 7 requires you to enter one.
To turn that option off, open <b>Network and Sharing Center</b> and, in the left column, click on <b>Change Advanced Sharing Settings</b>
Scroll down to <b>Password Protected Sharing</b>, select <b>Turn off Password Protected Sharing</b>, then <b>Save changes</b> at the bottom of the window. You can change other settings in that same window, such as Printer Sharing, etc..