Hopefully this will help those of you who are running Windows Server 2008 and are getting "Access Denied" errors when trying to run any command in STSADM.
- Make sure you're running the command prompt as an administrator (Right click command prompt in the start menu and select "Run as Administrator..."
- Make sure the username you're running under is set as farm administrator as well as a local administrator on the server.
- Make sure User Account Control is switched off (Control Panel > User Accounts). This is only really necessary if the above didn't work.
There was also a bug in older builds of MOSS 2007 (and possibly WSS 3.0?) that under specific conditions would cause Access Denied errors when running certain STSADM operation such as "import". These can be fixed by patching to SP2.