If you need support for a three-button mouse, there are a few options available. A popular freeware option is Clicker. And I use Middle Mouse Button, which is $10 after a trial period.

And if you have a Logitech 3-button mouse, you can use the middle mouse button for scrolling (like an Intellimouse "wheel") by simply changing the following registry value from 2 to 3:

HKEY_LOCAL_MACHINE /SYSTEM /CurrentControlSet /Services /SERMOUSE /PARAMETERS /NumberOfButtons=3

as per Logitech support.

X-Mouse Functionality

If you want X-Mouse support, you can simply change the following registry entry from 0 to 1:
HK_Current User/Control Panel/Mouse: ActiveWindowTracking=0

Tweak UI is one of the Power Toys which does X-Mouse, and more registry hacks for the mouse, automatically.