Ehsan,

On any XP Pro computer, check to see if Simple File Sharing (Control Panel -
Folder Options - View - Advanced settings) is enabled or disabled.  With XP Pro,
you need to have SFS enabled.

On XP Home, and on XP Pro with Simple File Sharing enabled, make sure that the
Guest account is enabled.  Enable Guest with Start - Run - "cmd", then type "net
user guest /active:yes" in the command window.

On XP Pro, if you're going to use Guest authentication, check your Local
Security Policy (Control Panel - Administrative Tools) - User Rights Assignment,
on the XP Pro computer, and look at "Deny access to this computer from the
network".  Make sure Guest is not in the list.