You need to go into the BIOS... also called CMOS. Set it up so your computer will "boot" off the DVD or CD drive and not your hard drive.
You usually get into the bios when it is first started by hitting the Del key. Some models, it's the F1, F2 or sometimes the F10 key... keep hitting the key until you get into the BIOS setup screen. You don't need an OS to get into the BIOS setup screen.
If you have problems, you need to go to your manufacturers website and find out how to get into the BIOS.
Also keep in mind that you will need "drivers" to be installed after you install the OS. You get these from the manufct website by searching the exact model.
You would probably be better off having a friend do it that has installed an OS from scratch before.
There are some exceptions to this:
1) Be sure your model will run WIN7. Also be sure if it will run the one you're installing, i.e., 32 bit 64 bit.
2) Be sure it's not an "upgrade"... if so you will have to use a different way since it should run from a machine already with an OS on it. OEM is the best version to install.
If you need a working computer, you will have a partition on your drive that will reinstall the original OS. This is usually involked with the F10 or F12 key... go to the manfact website to find out how. It will take the files from that partition and install your old OS on the main partition (Drive C).
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You're original OS was Vista of a particular type designed for a particular service. Sounds like a stolen computer. If so, they will track you with embeded serial number once you get online.
Anyway, you tell the new OS to delete the "partitions." You can have thousands of volumes (dynamic) which are usually used by big corporations and the government.
If this was an XP computer with dynamic disks then someone way up the ladder of technology was doing it for a special reason. Should never be used on a laptop.
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If all else fails, use Fdisk and remove all the partitions on your hard drive... then boot from the DVD.
http://en.wikipedia.org/wiki/Fdisk