Yes, it is a shell extension, a DLL which runs when you run Explorer.exe. You can disable it too. Go to Control Panel -> Internet Options -> Programs tab. Click Manage Add-ons and disable the Classic Explorer related addons. That will disable the Explorer addon.
The Preview pane can't be resized in Windows 8 because Microsoft stupidly merged the Details pane with it. You can use this app, OldNewExplorer:
http://www.msfn.org/board/topic/170375- ... win78-one/ to show the Details pane at the bottom. That will also allow you to make the preview pane smaller.
Disclaimer: Third party software recommended is only for your convenience. Use at your own risk. Classic Shell is not responsible for anything that happens to your PC from using it.