How are you trying to install? Worst case, you press F12 at boot up and that should allow you to select the DVD drive, boot the install OS from that, select the option to reformat the hard drive and then install. ??
In order to prevent an unbootable system INSTALLING A NEW OS or FORMATTING THE HARD DRIVE should be left for professionals or people who know exactly what they are doing. Just my 2 cents.
I have a Dell Inspiron 1525 which is almost the same as the 1545. I have upgraded/downgraded this machine several times. Put the DVD in the drive and see if it starts the W7???? or W8???? installation on its own. If not open the DVD directory and select the setup.exe program. If it still won't install leave the DVD in the drive and do a reboot and tap the F12 key as it is booting up which should let you select the DVD drive to boot from and start the installation process.