ET_Patch_2_60.exe will error out if you lack a registry setting that is created by the full ET installation. If you create the following registry key, the patch will run and you can select the path where ET is installed:
Windows Registry Editor Version 5.00
[HKEY_LOCAL_MACHINE\SOFTWARE\Activision\Wolfenstein - Enemy Territory]
"InstallPath"=""
You can save this text as a .reg file and double click it to add it to your registry.