I’m not sure what you mean by “focus a specific window”.
(Win)+n may be what you want; it will go to the nth task on the taskbar. If you want a certain combination (e.g.,
+5) to always go to a certain program, you should pin it to the taskbar. (If the task has multiple windows, hold
while you press n repeatedly, and it will cycle through the windows.)
Another answer is to invoke Windows Task Manager (e.g., by Ctrl+Shift+Esc), go to the “Applications” tab (if you aren’t there already), select the task you want to go to (click or cursor down or type the first letter of its name), and press Enter or click on “Switch To” or double-click on the task name.
Which window? That of a specific program always or do you want to press the shortcut, specify any program's name and then have its window focused? Plus what if multiple instances of the program are running? Also, I'm having a hard time figuring out the exact need for something like this. What is it you're looking to do actually? Maybe there's a better way to accomplish the same. – Karan – 2012-12-11T23:22:19.537