Make sure that the Software Firewall on each computer allows free local traffic. If you use 3rd party Firewall On, Vista/XP Native Firewall should be Off, and the active Firewall has to adjusted to your Network IP numbers on what is some time called the 
Trusted Zone (consult your 3rd Party Firewall instructions.
Windows 7 Work Network, 
http://www.onecomputerguy.com/windows7/windows7_sharing.htm
Vista File and Printer Sharing- 
http://technet.microsoft.com/en-us/library/bb727037.aspx
Windows XP File Sharing - 
http://support.microsoft.com/default.aspx?scid=kb;en-us;304040
Printer Sharing XP - 
http://www.microsoft.com/windowsxp/using/networking/expert/honeycutt_july2.mspx
Windows Native Firewall setting for Sharing XP - 
http://support.microsoft.com/kb/875357
Windows XP patch for Sharing with Vista (Not need for XP-SP3) - 
http://support.microsoft.com/kb/922120
Sharing between Windows XP and Mac - 
http://www.realifewebdesigns.com/web-resources/mac-pc-file-share.asp
When done hard reboot all Network's computers and the Router.
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Permission and Security issues with Vista/Win7, check the following settings.
 
All users that are allowed to share need to have account on
 all the computers that they are allowed to connect to.
 
Everyone is an account, it means a group of all of the users that already have an account and been established as users.
 Using the 
Everyone feature saves the need of configuring permission to each of the 
established users, it does not mean
 Everyone that feel that they would like to login. 
 
Users that do not have an account on the computer are Not part of the Everyone Group.
 If security on the LAN is 
Not needed and users are Not established, then switching On the
 Guest account provides semi-open configuration.
 
http://windows.microsoft.com/en-US/windows7/Turn-the-guest-account-on-or-off
 
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Point to the a Folder that meant to  to be share. Right click and choose properties.
 
In the properties. Click on the Security tab shown in the pic bellow to the right) and  check that the users and their permission (shown in the pic bellow  center and left) are correctly configured. Then do the same to the  Permission tab. 
This screen shot is from Win 7, Vista's menus are similar.
 
http://www.ezlan.net/Win7/Permission-Security.jpg
 In both the Security panel, and the Permission panel you have  to highlight each User/Group and examine that the Permission Controls  are checked correctly.
 
When everything is OK, Reboot the Network (Router, and computer).
 
* Note . The Groups and Users shown in the screen-shoot are just an example. Your list will look the way your system is configured. 
 
