What could be the reasons of getting "Access Denied" or "The remote machine does not exist or is unavailable" messages?

Check that you have local Administrator rights on the target machine. WMI will not function unless the account you are using has local Administrator rights on the machine you wish to monitor.

Check that DCOM is enabled on both the host and the target PC. Check the following registry value on both computers:

Key: HKLM\Software\Microsoft\OLE, value: EnableDCOM, should be set to 'Y'
Check that WMI is installed. WMI is present by default in all flavors of Windows 2000 and later operating systems, but must be installed manually on NT4 systems.

To check for the presence of WMI, type "wbemtest" into the Run box (Start Menu). If the WMI Tester application starts up, then WMI is present, if not, it must be installed.

Ensure that WMI permissions have been set correctly.

On a Windows XP Pro computer, make sure that remote logons are not being coerced to the GUEST account (aka "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, without quotes). 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.

Also on an XP computer running SP2, configure the firewall to allow remote administration. To do this, open a command prompt and type: netsh firewall set service

If you have other internal firewalls on your network, you may have to configure them to allow WMI messages.
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.
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
Windows Management Instrumentation
Windows Management Instrumentation Driver Extensions
WMI Performance Adapter

Tip: For more information about troubleshooting WMI, please see the Microsoft WMI troubleshooting site.