Hi there...
Most of us have installed 2 OSs on our System and using Windows as default but most of the times the OS which is installed after be the Default OS so we need to select OS at booting time.
Here I am sharing How you can set Windows back as a Default OS....
1) You’ll need to right click on Computer and select Properties
2) Next, click Advanced System Settings
3) Now click on the Settings button under Startup and Recovery
4) And just select the operating system you want to use.
5) You can also change the waiting time to display list of OSs.
till then...
njoy the simplicity.......