With a dual boot (Windows and Ubuntu linux) system, Ubuntu installs GRUB (GRand Unified Bootloader) which allows you to select which OS you want to boot with. The default is Ubuntu. However, sometimes you’ll have a system that you’d like to have default to Windows (like my wx-aprs computer).
The latest distribution of Ubuntu (9.10) has changed the way GRUB works and the Ubuntu wiki explains what you need to do if you want to change the default OS GRUB will select when going through the boot process.