My PC will be repaired, so I have to create a recovery DVD. How can I do this?
Do you mean that you want to back up your entire drive so that you can restore it as it was before you sent it in for repair? Or do you want an Ubuntu installation disk with your particular set-up?
To back up your entire disk, the best solution is a product called RedoBackup. It is a user-friendly product, but you have to install it first. Be aware that your installation will probably not fit onto a DVD, and so RedoBackup will back up instead onto different media; the recommended medium is an external hard drive, or a hard drive on a different computer over the LAN. If you do not have an external hard drive, or access to another computer over your local LAN, this solution is not for you.
To install RedoBackup, first download it. To install to a USB stick, install UNetBootIn (if I remember correctly, it is in the Ubuntu Software Centre), and use UNetBootIn to create the USB using your downloaded RedoBackup. To burn to a CD, use the Brasero Disk Burner in Ubuntu.
As a precaution, please do your usual daily backups before you send in your computer!
To back up your hard drive, boot from the RedoBackup CD or USB (whichever one you made). Follow the instructions to back up.
To restore your computer when you get it back, book from the RedoBackup CD or USB and follow the instructions to overwrite your hard drive. Warning: This procedure will erase everything on your hard drive before it restores (which, presumably, is exactly what you want).
If you cannot use RedoBackup, the best solution in my opinion (others may disagree) is:
When you get your computer back: