WMI error: 0x800700a4 - The @wmi_query could not be executed in the @wmi_namespace provided
Fix #1
In some cases I was able to solve the problem by restarting the WMI service "Windows Management Instrumentation", and then re-running the script. This did not help in every case however. 

Fix #2
On both of the SQL 2008 R2 servers, I noticed that the WMI service ("Windows Management Instrumentation") was running as Local System, so I checked to see if it had login rights on the SQL instance. It did indeed already have a login "NT AUTHORITY\SYSTEM", with no particular rights granted to it. I gave it sysadmin temporarily, and was able to run my script successfully. I then removed sysadmin. and my WMI alert continued to work.

Fix #3
Another possible reason is the account that runs the Windows service "Windows Management Instrumentation" is disabled as a SQL login. (If you are running SQL 2012+, look for the login 'NT SERVICE\winmgmt'). (Source: Blog by 'rahmanagoro' )