If double clicking on a folder on the right hand pane of explorer opens a new window all the time, here is the fix. I know this works for XP.
Open explorer
Go to Tools > Folder Options... > File Types (tab)
Select "File Folder" and click on "Advanced" button.
Add a new junk entry by clicking "New".
Enter values "To DOS" and "cmd.exe" in the two text boxes and press "OK".
Then select "To DOS" and click "Set Default" (This is very important)
Click "OK", "OK".
Close explorer.
Open regedit (Start > run > regedit > OK)
Search for "To DOS".
Delete this node from the "shell" folder (HKEY_CLASSES_ROOT\Folder\shell).
Close regedit and restart explorer and the problem should be fixed.