Share Files and Directories on Windows 7 with Windows XP

Solution 1:
  1. Goto Network and Sharing Center
  2. Advanced sharing settings
  3. In the bottom Password protected sharing, click on  Turn off password protected sharing
  4. Save changes
Solution 2:

  1. Click on Properties over Directory shared.
  2. Shared properties
  3. Click on Permissions then Add
  4. Find Network and add to list.
  5. Save changes.

References:

Share Files and Directories on Windows 7 with Windows XP.
[1] http://support.microsoft.com/kb/980458/es