This should do the trick:
1. Go to [Control Panel -> System]
2. Switch to Tab "Advanced"
3. In section "Startup and Recovery" click "Settings"; a new window appears
4. In section System Start up, there's an option
"Time to display list of Operating Systems:"; set the value in the field next to it to 1 second.
5. Confirm changes by clicking "OK" in this windows and in Systems window.
List of Operating systems will probably flash too fast for you to even notice it (and even if you do, it will only happen for a fraction of a second).
You COULD totaly remove the entry of the first Operating System (by clicking "Edit..." instead of modifying the value) but I don't want you do mess something since you can render your system unbootable which is a totaly new problem (but not unsolvable).
This should satisfy you.
The Borg Collective, rendering assistance.