camallison: the statement "If this service is stopped, most Windows-based software will not function properly." is incorrect. Bill Mullins and others are wrong as far as home users are concerned. If you run an Enterprise network, then that's a different matter.
The WMI service is all about collecting management information about applications. It has nothing to do with the actual functioning of Windows or applications.
For the home user there is very little risk that an application will not work. I run without WMI on my XP/Pro/SP3 and have no problems with any application including Firefox, IE, Microsoft Office, Agnitum Firewall, Avast anti-virus, many AV applications, Skype etc.
toulouse: It's easy to just try it. If you get problems, turn it on again. The mechanism is go to Start > Control Panel > Administrative Tools > Services. Then go to the WMI service, right click and in the Properties you can stop it, and disable it. If you want to set it back the way it was, just do the same again but in the Properties, set it to Automatic.