I seem to recall there being a way to do this using the bcdedit command, but there is another way as well. The easiest way is to click on your start menu icon, then click on control panel. From there make sure the view is set to small icons. Click on System. From within System, click on the left pane on Advanced System Settings. Go to the advanced tab, and select Settings under the Start Up and Recovery Section. From there you can change both the default OS and the amount of time to show the list of OS's in Boot Manager.
Default Operating System - Change Default Boot OS
I've attached some pictures to help you out.
In addition, you could also download a great free program known as EasyBCD. (Find it
here.) It's very easy to use, and will help you set the default OS, as well as add new entries to Boot Manager and change their names. I found it very useful when I was using different Linux distros as I would launch through Windows Boot Manager and select the correct OS. Grub was just too ugly for me.