How to list Explorer extensions and disable them?

My personal favorite is Autoruns from Sysinternals (Microsoft). It goes well beyond just shell extensions and covers tons of areas where third party code can run.

Assuming you've done a virus scan and it didn't catch it, you might want to watch this excellent video from Mark Russinovich on advanced malware cleaning techniques.

ShellMenuView is a small utility that display the list of static menu items that appeared in the context menu when you right-click a file/folder on Windows Explorer, and allows you to easily disable unwanted menu items

Shell Extensions are in-process COM objects which extends the abilities of Windows operating system. Most shell extensions are automatically installed by the operating system, but there are also many other applications that install additional shell extension components. For example: If you install WinZip on your computer, you'll see a special WinZip menu when you right-click on a Zip file. This menu is created by adding a shell extension to the system.

The ShellExView utility displays the details of shell extensions installed on your computer, and allows you to easily disable and enable each shell extension.

CCleaner also has a feature to clean up the context menu.

It's found under Tools -> Startup -> Context Menu