Even with UAC disabled, are you using an administrative command prompt?

The security token for administratively running IISRESET isn't there under a normal command prompt even if you've moved the UAC slider to "Never Notify".

You can follow these steps:

  1. Open PowerShell
  2. Type Regedit then hit enter
  3. Browse to HKEY_LOCAL_MACHINE\SOFTWARE\MicrosoftWindows\CurrentVersion\policies\system
  4. Double click on EnableLUA
  5. Change the Value data to a 0
  6. Click OK, then reboot the server

To completely disable UAC, and then try running IISRESET from a normal cmd prompt, but it isn't good practice to completely disable UAC on any server.