the window will only dock IF the mouse cursor touches the edge of the screen.
I'd either use Win+tab or alt+tab to get to what ya want...
you could also shake the window that will minimize all but the window you want
OR you could do win+D
but to really answer your question
If you want to disable the automatic arrangement (Aero Snap) feature, use these steps:
1. Click Start, Control Panel
2. Click Ease of Access and click Ease of Access Center
what ya want is in there.
EDIT:
3. Click one of the following options:
* Make the mouse easier to use
* Make the keyboard easier to use
* Make it easier to focus on tasks
4. Place a checkmark near the following option:
Prevent windows from being automatically arranged when moved to the edge of the screen
5. Click OK, and close the Ease of Access Center