By default, installing Ubuntu on top of Windows OS will set Ubuntu as the default OS. To change the boot up default section, please follow the Ubuntu documentation.
https://help.ubuntu.com/9.10/switching/dualboot-custom.html
Upgrading Ubuntu tends to add additional entries with new version number into the boot menu. If moves the entire Windows OS block ahead of Linux OS, the block will be removed at the next version upgrade so it is best to avoid.