You need to enter the BIOS. When you start your PC, you'll probably see a screen which says something like 'To enter Setup, press Delete'. Follow this instruction. (Different computers allocate different keys to this function but Delete, Esc, F1 and F2 are the most common ones).
When you get to the BIOS screen, remember that your mouse driver won't have loaded, so all navigation is achieved by using the arrow keys and 'Enter'.
The BIOS screen on your computer won't look the same as on mine, so it's impossible to give you precise instructions. However, you should find on-screen instructions available. (e.g. F1=Save and exit, F2=Exit without saving, etc). To go back to a previous screen, you usually use the 'Esc' key.
I'm sorry that what I've written is a bit vague but, if you play about a bit, you should soon locate the screen which determines the order that your PC examines drives for boot instructions. It's usually best to put your CD drive (or your floppy drive) first. That means that, if things go wrong, you can boot your PC from a Windows disk (or from a boot floppy). Then set your internal hard drive to be the second boot device.
As I've stated, you might need to 'play around' to locate the correct screen but, as long as you don't hit the 'Save' key, you can't do any damage to your BIOS. Once you've changed the boot order, hit the 'Save and exit' key and restart your PC.
Chris