Ubuntu installer won't boot from DVD

I'm trying to reinstall ubuntu-13.04-desktop-amd64.iso on a Toshiba Satellite P855-335, but Ubuntu installer doesn't boot from the DVD.

I've set up the boot priority properly and when the laptop restarts I hit F12 and choose the ODD. The laptop starts reading from the DVD, but the installer does not start and instead runs the existing Ubuntu installation from the HD.

I'm happy to supply further information if required.

Additional information:

Description: Ubuntu 13.04
Release: 13.04

ProblemType: Bug
DistroRelease: Ubuntu 13.04
Package: installation-report 2.49ubuntu1
ProcVersionSignature: Ubuntu 3.8.0-26.38-generic 3.8.13.2
Uname: Linux 3.8.0-26-generic x86_64
NonfreeKernelModules: nvidia
ApportVersion: 2.9.2-0ubuntu8.1
Architecture: amd64
Date: Sun Jul 28 17:32:24 2013
InstallationDate: Installed on 2013-03-16 (133 days ago)
InstallationMedia: Ubuntu 12.10 "Quantal Quetzal" - Release amd64 (20121017.5)
MarkForUpload: True
PackageArchitecture: all
SourcePackage: installation-report
UpgradeStatus: Upgraded to raring on 2013-05-04 (84 days ago)