We can't tell you why it's failing without you providing significantly more information--the exact make and model of your Mac Mini, what file you've downloaded, if you've verified its integrity (see below), precisely how you're trying to burn it and how you're trying to write it to a USB flash drive, with what utility, what settings, (for the burn) what speed, whether it's DVD+R or DVD-R and the make and model of your DVD drive (which you can usually get in the Windows device manager -- Start > Run... >
devmgmt.msc; and on the Mac, in Apple System Profiler or, better, in the manuals).
I can give you some tips, though:
You're installing on a Mac, so if you're installing the 64-bit version of Ubuntu, use the Mac image. (This is not a CPU difference; rather, Macs can have trouble booting the usual, multi-catalog ISO images.) So if you're installing the 64-bit version, make sure you use the ISO image with
amd64+mac in the name--not just
amd64. (That's this one.)
Now that is assuming your Mac Mini has an Intel processor. But if you have an old Mac Mini, that is not the case. If you have a Mac with a PPC processor (probably a G5), you'll have to use the PPC version of Ubuntu (available here). On the other hand, if your Mac has an Intel processor, make sure you are not using that version.
- MD5SUM the ISO image; make sure it's good.
- Make sure you're burning the ISO to the DVD properly (see the Windows and OS X instructions). In particular (and while you may know this many people do not): don't make a DVD with one file on it such that this file is the ISO, and don't unpack the ISO image and burn its contents to DVD.
- Burn the DVD at the slowest possible speed. This can often reduce errors and lessen the impact of any damage or incompatibilities between the DVD media and your drive.
- Check to see that your DVD media is not damaged before you start the burn, and make sure it is compatible with your burner in both type (there's DVD-R and DVD+R, and there's single and dual layer for each) and speed (discs have a maximum speed).
- Make sure the DVD media are of a type compatible with the machine on which you're trying to install. Remember that many computers are capable of reading media they cannot boot.
- You have not said how it fails. If it doesn't boot at all--the same as if you'd put an unbootable data DVD or a blank DVD in the drive--then try booting it on the other computer (the Windows XP machine). Hold down Shift and select Check disc for defects--see if there are errors. If any files are shown to be bad, that means it's a bad burn, you shouldn't attempt to install from that DVD, and corruption elsewhere on it may explain why it didn't boot at all on the Mac. On the other hand, if it boots on the Mac but doesn't get very far then try to run Check disc for defects on the Mac. (This doesn't just test the DVD installation medium but also will indicate failure if your computer is unable to read the disc.)
For trying to install from USB:
- Use the correct ISO image (as explained above).
- MD5SUM the ISO image (as above).
- Make sure you're writing the USB flash drive correctly. This is likely what's going wrong. A USB flash drive created using UNetbootin or the Universal USB Installer will not boot on a Mac. You must instead open Applications > Utilities > Terminal in OS X and use this technique.
- As with the DVD, try booting the USB flash drive on the PC if it cannot boot on the Mac, and run Check disc for defects (see above for more detailed explanation). In spite of its confusing name, this boot menu option applies equally to CD/DVD and USB media.
Hopefully that has enabled you to solve the problem. But if not, you can edit your question to describe in detail what happened when you tried all this (and also what happened originally--exactly how you'd created the DVD and USB media, how you tried to boot it, what happened when you tried to boot it, and so forth).
- For people using this answer other than this question's author, you can post a new question, providing this information. (If this and any other answers here don't help you, you should probably not assume you have exactly the same problem.)