1. Turn off Firewall on the client computeris mandatory (under control panel)
2. Make sure the destination computer is available: Try to ping the other computer.  
3. Include the domain name when entering username field (i.e. Domain\Administrator)
4. Use another administrative account: Try the local computer’s account. Try a different domain account.
5. Turn off Simple File Sharing on the client computer(under Folder Options)
6. Try to use the IP address instead of the computer name, or vice versa.

1. Ensure the RPCSS service is running on the server. Go to Start Menu, click Run, type in the following line:

net start rpcss

Test to see if this resolves the issue. If the issue still occurs, continue to the next step

2. Make sure that the username and password you have supplied for each computer correspond to an administrator account on the target computer. That account must not have a blank password. 

3. Check that DCOM is enabled on both the server and the client PC. Check the following registry value on both computers:

HKey_Local_Machine\Software\Microsoft\OLE, value: Enable DCOM, should be set to 'Y' 

4. For Windows XP SP2, configure the firewall to allow remote administration. To do this, go to start menu > run > type in:

netsh firewall set service RemoteAdmin
If you would rather use the Group Policy editor than the NETSH commands above, use the following steps in the Group Policy editor (Gpedit.msc) to enable "Allow Remote Administration" on the client computers. 

1. Under the Local Computer Policy heading, double-click Computer Configuration.
2. Double-click Administrative Templates, Network, Network Connections, and then Windows Firewall.
3. If the computer is in the domain, then double-click Domain Profile; otherwise, double-click Standard Profile.
4. Click Windows Firewall: Allow remote administration exception.
5. On the Action menu, select Properties.
6. Click Enable, and then click OK.
5. Check that WMI is installed. 

6. On a Windows XP Pro computer, make sure that remote logons are not being coerced to the GUEST account ("ForceGuest", which is enabled by default computers that are not attached to a domain). To do this, open the Local Security Policy editor (e.g. by typing secpol.msc into the Run box). Expand the "Local Policies" node and select "Security Options". Now scroll down to the setting titled "Network access: Sharing and security model for local accounts". If this is set to "Guest only", change it to "Classic" and restart your computer. 

7. If you have other internal firewalls on your network, you may have to configure them to allow WMI messages.

8. Even if you are not knowingly running any firewall software, bear in mind that big-name antivirus solutions such as those produced by McAfee and Symantec often contain their own firewall functionality. If such software is not properly configured to allow WMI traffic, then this may be the cause of the problem.

9. Make sure that no remote access or WMI-related services have been disabled. On an XP machine, the following services should be running (or at least allowed to start on demand):

COM+ Event System
Remote Access Auto Connection Manager
Remote Access Connection Manager
Remote Procedure Call (RPC)
Remote Procedure Call (RPC) Locator
Remote Registry
Server
Windows Management Instrumentation
Windows Management Instrumentation Driver Extensions
WMI Performance Adapter
Workstation