I wrote the Ubuntu ISO on a CD and restarted my computer, but "Press Any key To Boot From CD Or DVD ... " didn't appear! What should I do? Is it because (I guess) the file maybe is corrupted? Because when I see it's propeties windows, It is written 687 Megabytes!!!! (V 12.10) Thanks
Ubuntu 12.10 doesn't fit on a CD. You need a DVD or a USB drive. The iso is around 800 Mb.
If your computer can boot from a USB, like all computers less than 6 years old, you can download Unetbootin and create your bootable USB drive.
To boot from the USB, on many computers you have to press F12 or some other key to show a boot menu and then you can choose the USB to boot from. Otherwise, go in thr bios setup (by pressing F2 or Esc in most computers), and change the boot order respectively.
If you have important files on your disk, make sure you keep a backup of everything. Installing Ubuntu carries a small risk of data loss, especially if you're not careful enough or do not have any experience with Linux.