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#11
The part in bold is still a problem as Homegroups is simply not made to work with Op systems other than Windows 7. You need to leave the Homegroup from the Win 7 machines first. Homegroup - Leave
After that set up a Workgroup by following the tutorial already posted. If you are running any type of A/V software you may need to uninstall it completely. You can reinstall the A/V's later after your network is up and running.
After you set up the workgroup and all permission and sharing settings are correct, if you are still having problems seeing those machines there are a couple more things we can try.