Contributed by B@rt.
If your mouse does not have three buttons, hold down the ALT key and use the left mouse button to simulate the middle mouse button.
Contributed by Zycho.
If Blender loads an old blend file each time you start, and/or ctrl+x doesn't get you the default startup screen chances are you've saved over the the user defaults. There are two ways to fix this:
1. start Blender and delete everything you don't want in the scene. Once you've created a nice startup screen you can save your user defaults by pressing ctrl+u.
2. To reset your user defaults to the original user defaults you need to locate a file called .B.blend. Delete it and Blender wil automatically create a new one with the original defaults.
Under Windows, the .B.blend file is now located in your 'Application Data' folder as recommended by MicroSoft. But as it turns out this folder is hidden by default on Windows and search queries won't find this file....
So you have to Activate:
Tools->Folder Options->View->Show hidden files and folders.
The process is described in detail here:
https://biorobots.cwru.edu/server/howto/seehidden/