Windows 7 - How can I make a bootable DVD?

I'm confused as to what you have in hand.

If you have a Microsoft OS installation DVD, it's copyable.

If you wish to save a copy of a DVD to a hard drive, perhaps the best way is to use a utility to save an .iso of it. The .iso is an image of the DVD, preserving all of its contents in a single file.

If you have a pre-installed version of Windows 7, the PC manufacturer probably included a utility to make system restore disks. In my opinion, that's much less useful than having a Microsoft installation DVD, but it's better than nothing.

Windows 7 includes drive imaging software. You could back up your current installation with that. (You'd need a separate partiton on the hard drive, another internal hard drive, or an external drive.)

It may be possible to reconstitute an install disk from a running Windows 7 installation, but I can't recall reading of anyone having done that.