1. I have some ideas, but some clarification on a few of the details may help. When you went to your friend's computer, was the DVD you burned using the same iDVD project file copied from your computer? Does "without any problem" mean that the DVD played correctly in a DVD player? Did you make the "Toasted" copy on your computer or your friend's computer? So you're saying that the DVD you burned with your friend's computer played correctly in a DVD player, but the "Toasted" copy of that DVD did not?
~ ~ ~
2. What we do know is this: It's probably not your DVD burner, because it burned a DVD, not because it burned a CD. DVD burning is a different mechanical act than CD burning, one that uses a different laser. There are 4 laser beams used by most CD/DVD burners: CD-read, CD-write, DVD-read, and DVD-write. Each one uses a different amount of power, and selecting faster reading/writing speeds will use even more power. So, a broken CD/DVD burner might still read (play) DVDs fine, but it won't write (burn) DVDs at all, or it might burn DVDs correctly, but it can't burn CDs without errors. In other words, just because 1 laser beam works doesn't mean the other 3 do.
~ ~ ~
3. The culprit is probably either your blank DVDs or iDVD. To verify the problem isn't with your blank DVDs, try burning one of your own blank DVDs using your DVD burner and your friend's computer (if you haven't already). And then right afterwards, burn one that came from a different pack of blank DVDs, again using the same burner and computer. And by the way, if the DVDs you're dealing with are double-layered DVDs (a "DL" will follow their name, as in "DVD+R DL" or "DVD-R DL"), peculiar issues can arise when using them. They are useful in the right conditions, but single-layer DVDs are generally a better choice.
~ ~ ~
4a. If it's not the blank DVDs, the culprit is probably within iDVD. To verify that, use iDVD to export your iDVD project to a disk image, instead of burning it to a DVD, and then test it inside Apple's "DVD Player" app. (See "USING DISK IMAGES WITH iDVD" below if you would like a more detailed explanation.)
~ ~ ~
4b. If the disk image does play correctly inside Apple's "DVD Player" app, use Apple's "Disk Utility" app to burn that disk image to a blank DVD. Then, verify that the freshly burned DVD plays correctly inside Apple's "DVD Player" app. This won't fix your problem, but it does present you with a useful workaround. Unfortunately, if that freshly burned DVD does not play correctly inside Apple's "DVD Player" app, and you're sure that your blank DVDs aren't the problem, then your computer doesn't like that DVD burner. Get a different one.
~ ~ ~
4c. However, if the disk image doesn't play correctly (or even open) inside Apple's "DVD Player" app, then the problem is inside iDVD. To see if the problem will require reinstalling iDVD, you will first need to recreate your iDVD project again from the beginning, this time using a simpler DVD menu. Starting with a new iDVD project, keep your DVD menu really basic, with no slideshows and only a few items on each menu page, and only use a few menu pages. Then, use iDVD to export this new iDVD project to a new disk image, and open that new disk image in Apple's "DVD Player" app. If it plays correctly inside Apple's "DVD Player" app, then your original DVD menu design was the problem, not iDVD - problem solved. But, if the new disk image doesn't play correctly (or even open) inside Apple's "DVD Player" app, I would either try the whole thing again with an entirely different iMovie project (to be sure the problem isn't your source material), or just reinstall iDVD.
~ ~ ~
5. USING DISK IMAGES WITH iDVD: iDVD allows you to export an iDVD project into a "disk image". A disk image is like a special folder which contains all of what gets burned on to a DVD (or CD), and it uses the .dmg file extension. Once a disk image is created, it will need to be opened (which will "mount" it in the Finder) in order to access it. Once it is opened, if you then open Apple's "DVD Player" app, any mounted disk image (that was made in iDVD) should automatically start playing in Apple's "DVD Player" app, as if it was a regular DVD. While you watch it using Apple's "DVD Player" app, verify that the DVD menu pages and the DVD video/audio are all visible and of an acceptable quality. Next, use Apple's "Disk Utility" app (in the Utilities folder, which is inside the Applications folder) to burn that disk image to a blank DVD; the disk image must still be open (mounted) in order to be easily visible from within Apple's "Disk Utility" app. When you're done with the burning, you'll need to eject (unmount) the disk image from within the Finder, as if it was an actual DVD (or CD). Lots of people, including myself, regularly use disk images to test their finished iDVD projects before burning them to DVDs.
