Carlos,

What OS are the 3 computers running - XP Home, XP Pro, a mixture, other?  This
makes a difference.

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 properly set on each computer.

On XP Pro with SFS disabled, check the Local Security Policies (Control Panel -
Administrative Tools).  Under Local Policies - Security Options, look at
"Network access: Sharing and security model", and ensure it's set to "Classic -
local users authenticate as themselves".

On XP Pro with SFS disabled, if you set the above Local Security Policy to
"Guest only", enable the Guest account, using Start - Run - "cmd" - type "net
user guest /active:yes" in the command window.  If "Classic", setup and use a
common non-Guest account on all computers.  Whichever account is used, give it
an identical, non-blank password on all computers.

On XP Home, and on XP Pro with Simple File Sharing enabled, make sure that the
Guest account is enabled, on each computer.  Enable Guest, with Start - Run -
"cmd", then type "net user guest /active:yes" in the command window.  Ensure
that the password for Guest is blank, with Start - Run - "control
userpasswords2"; select Guest, click Reset Password, click OK without entering a
new password.

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.  Look at "Access this computer
from the network", and make sure that Everyone is in this list.