You don't need to boot to your desktop. you need to boot to your W7 CD.
If CD drive is not your first boot priority in your bios, you need to enter your bios to make it so. When that is done and you boot up you will see in your boot post "press any key to boot from CD". Do so and your W7 installation CD will boot, not your desktop.
From there it's just. a matter of following the prompts. If you don't know how to enter your bios, please let us know. However, most PC's are already set with CD as first boot priority. If you don't see "press any key to boot from CD" it isn't.
Cheers,
Lee
Because of the wide variety of computer and BIOS manufacturers over the evolution of computers, there are numerous ways to enter the BIOS or CMOS Setup.
It should say in your boot screen what to press to enter your particular BIOS. It can be anything from F2 to F12. However, one method that should work with all PC's is to press your boot-up button and then immediately start pressing your "delete" button over and over again until your BIOS shows up.
Once into the BIOS be careful about what you change. You cannot use your mouse to manouver around but must use your arrow keys, enter key and escape key. It's all fairly logical once you understand that.
Your looking for the "boot sequence" entry (press "enter" to view what's in your main BIOS headings and "escape" to exit them). When you find "boot sequence" press "enter" and use your arrow and "enter" keys to change it to CD as first boot. Then the "escape" key should be pressed and navigate with the arrows to "save changes and reboot". Then press "enter".
Make sure you have your W7 CD in your optical drive if you can when booting into Bios because when you set the boot order and get to "save changes and reboot", you want the CD in the optical drive so you can "press any key to boot from CD".
Alternativly, press boot and immediately put yor W7 CD in your optical drive. You should still get the "press any key to boot from CD" message. If not, leave the disk in the drive and reboot.