Your DVD drive may not be on your boot menu ahead of the hard drive. If this is the case, you won't be able to boot from _any_ DVD (test with the Win7 DVD?).
Either go into BIOS and put the DVD drive ahead of the HDD in the boot sequence, or press the indicated key during the first BIOS screen during power-on (usually F8) and you will be presented with a list of boot devices at the end of the BIOS sequence. Pick the DVD drive.
I hope that it's that simple.