I have installed ubuntu alongside windows 7 in a brand new computer and it worked fine for some days. Eventually, Windows showed some problems, but after a safety mode initialization it was working again. Then, Ubuntu cracked and I couldn't use it. After that, I have reinstalled Windows using the recovery CD and now I can't boot from the CD. I put the CD with the Ubuntu image on the drive, turn the computer on, press f12 to choose the drive for the boot, select to boot from the CD and then it automatically starts Windows, ignoring my choice. How can I substitute Windows for Ubuntu?
Try doing off a Live USB.
When you burn the CD you should not be treating the iso file as a file. If you just copied the file to the CD it is not a bootable CD. Most CD burners have an option to burn a disk image file. The disk image is stored in an ISO file. You have to burn the image to CD, not the iso file. If you double click on the iso file, it might default to exactly what you need.
Once burned, put the CD into the CD drive. If the CD has just the ISO file, then you did not burn the image correctly, if there is an assortment of files, then you probably did it right.
When you boot, go into the bios (typically with F8, F12, DEL, or other special key). In the BIOS you can view the boot order. You can change the order to give priority to the CD or even the USB. BUT, when trying to boot, if a choice does not present bootable media, the BIOS will step to the next option.
If you display a boot menu and pick a device that does not have a bootable system, the BIOS will then sequence through the list defined in the BIOS.