Hi versky
Depending on what bios you have the keys may be different, so if first solution doesn't work then set the boot drive to the dvd drive inside the bios.
1. Reboot and then when BIOS screen comes up press F8 repeatedly. After some seconds a screen should pop up asking you which drive you want to boot from, and there you can select the dvd drive.
If the above doesn't work:
1. Restart and press DEL key repeatedly when the screen appears (could also be F1).
2. Inside bios there should be a menu selection named 'Boot options' or 'Boot menu'.
3. Once you locate that one set the 'first boot drive' to be the dvd drive.
4. save and exit bios, it should now boot from the dvd.
5. Remeber to set the drive back to the original once you have installed windows (or when windows boots during the install process).