You get into your BIOS by pressing a key at boot-up. This differs depending on brand of computer. With most generic computers, it's the Delete key. Some computers have other key combinations. When the computer is booting, you'll see a black screen with lettering usually and at the bottom of the screen it will say something like "Press Del to enter Setup." Or it could be an F key or a combination of ctrl+alt+esc.
Once you're in there, you have to use arrow keys to navigate and other keys to make changes. The instructions will appear somewhere on the screen itself, such as using + and - keys to change a setting. What you're looking for is boot order - which disk boots first. It can be CD-Rom, hard drive, or floppy. You can change the boot order so that it will boot from the CD-Rom first. It may take a little practice to get it figured out. You must exit saving your changes.
When setting up your disk, you go for a clean install of Windows, but you have to choose where to put it. You need to see what's on the hard drive. If you are doing a clean install, it helps to understand what you're doing. If you have an H: drive, is it an actual hard drive or another partition on a single drive? If one partition has the operating system, and the other has data that you do not want to lose, you would not mess with the partition that has data. You will lose all your data and programs on the drive you are going to clean-install! You should always have all your data backed up before you do anything like this. That means favorites and email accounts and anything else you really want to keep.
I like this site
because it has a graphical representation of what you'll actually expect during an install.