Windows 7 will not let you span a window across multiple monitors, it always snaps it instead

Problem:  Windows will not let you span a window across multiple monitors.  If you try, it will always "snap" the window to a single monitor.

While this can be handy in some instances, it actively prevents you from stretching a window across multiple monitors.  There are lots of reasons why you would want to do this.

Solution:  You may be using an AMD graphics card driver that uses the Catalyst Pro Control Center.  If so, go to Catalyst Pro Control Center / Hydravision and check "Allow spanning across displays" in the Extended Display Window Control section.

Aside from that, I do not know.  Check your graphics card drivers along with any desktop managers you might be using to see if there is a similar feature preventing window spanning.