If you click on the D: drive letter in Windows Explorer, do you get an error that there is no disk in the drive, or does it show an empty disk?
Did you try ejecting the DVD and making sure you hadn't gotten any dust or fingerprints on it? Even just inserting it again might help (though I've found laptop optical drives to be much more finicky that way than desktop drives).
Have you had any problems with other CD/DVDs?
It is possible your optical drive is failing, or just has a problem with this particular DVD. You could try rebooting into safe mode and see if the computer recognizes the DVD, to eliminate the possibility of a software problem/conflict (though that is unlikely). A Linux Live CD/DVD should also work if it loads to RAM and makes the drive available (assuming you have one handy).
Can you borrow a USB optical drive, or something? A new DVD-RW drive would be pretty cheap, and a relatively easy install, but perhaps a bit much if this is the only disk your current drive has a problem with.
