First make sure both machines are members of the same Workgroup which by default is named Workgroup.
On the Windows 7 machine go into Control Panel \ All Control Panel Items \ Network and Sharing Center then click on Change advanced sharing settings.
You will want to verify the following settings under Advanced Sharing Settings for the Home or Work and Public profile.
If you want any user to have access the public shares turn off password protection.
or just go http://www.howtogeek.com/howto/windows-7/share-files-and-printers-between-windows-7-and-xp/