Many customers have a small home network which connects 2 or more computers. Often happens that one or several computers are unreachable for other computers even though there is no password protection on the blocked computer. This is usual trouble for WinXP users.
To fix that trouble user should set up the blocked computer with following secure settings:

1. Open control panel (Start->Settings->Control Panel)
2. Double click on 'administrative tools'
3. In the 'Administrative tools' window double click on 'local security policy' icon.
4. In the tree view of security settings choose: SecuritySettings->Local Policies->User Rights Assignement
5. You will see the list of access rights in the right frame of the security settings window.
6. Find 'Access this computer from the network' point and click on it.
7. You will see small window, which displays computers/groups which are allowed to access this computer.
8.Click the 'Add User or Group' button in this window.
9. You will see 'select user or groups' window.
10. click 'Advanced' button.
11. click 'find now' button.
12. After short time you will see a list of possible computers which can be added to allow list of this computer.
13. add needed computer/group
14. Click 'OK' button
15. Click 'OK' button on the 'Access this computer from the network' window.
16. Close security settings and control panel windows.

After this is done you may need to reboot the computer, but may be all will work without reboot (depends on system configuration).
Try to access the locked computer from other computers. All should work now.