To "boot" a dvd you don't go into windows at all.
You need to have the cdrom drive FIRST in the Bios BOOT ORDER.
It
might already be .
Put the Vista dvd in the drive and start the PC.
If you get a message "press any key to boot from cd rom /dvd drive" - then press any key.
If you didn't get that message, you may have seen during POST ( power on self test - that is the white writing on the balck screen when you start up the pc) something like this:
F2 for setup ( or del , or some other key - it varies per machine)
F11 for boot menu.
If you have the boot menu option - press the key it tells you - you should get a pop up boot menu. Put the dvd in the drive, close it. Arrow to cdrom/dvd drive and press enter.
Then you will get " press any key... " message.
If you don't have the boot menu option , you need tp press the key it tells you for SETUP.
That takes you into Bios settings .
In there you will find you can set cdrom /dvd drive first in the BOOT ORDER .
It will tell you what keys to press to Save and Exit - often F10.
Then start the machine with the dvd in the drive.
Follow instructions here:
Clean Install with a Full Version of Vista - Vista Forums
But at step 10, DO NOT select drive options (advanced) and DO NOT format the partition.
Instead, when you see the first screen showing your partitions, simply click the one you want to install onto ( to highlight it), then click Next.