When your computer is booting, check for a short message on the screen, about how to enter the bios or setup. It'll probably be the delete key, but it varies by exactly what bios and version you are using. The bios is where you change the boot order. Ubuntu won't overwrite windows without telling you about it first. If you are really nervous about it there are programs that will install linux as just another file on your windows filesystem, no chance of an over write there. It'll be a little slower they say, but with modern hardware it's difficult to notice. Here is an Ubuntu installer that works that way.
Let us know how it works out for you...
P.S. Oh, it's a really good idea to have backups in any event, especially if you are installing a whole other OS on your machine. Check out partimage too, just to be extra safe.