First step run F12 preboot diagnostics. Run all checks including any extended tests
I suspect you may have a failed HDD or a faulty RAM module.
Let us know whether the tests pass or not.
Regarding Windows Installation Media the XPS 8100 is an Early 2010 Model meaning all XPS 8100's are out of warranty. Windows 7 has also Reached End of Mainstream Support and End of OEM sales. Microsoft only allow Dell to provide Windows 7 installation .isos for new (2015-2016) systems which are sold with Windows 10 Pro OEM Downgrade Rights.
Microsoft have listened to our feedback on the forums regarding Windows Installation Media. I've been highlighting issues such as yours for quite some time. With Windows 10 you can Download the latest installation .iso direct from Microsoft. You may clean install Windows 10 with your Windows 7 OEM Product Key. Ignore any remarks you read online about a 1 year promotional period, this was extremely poorly worded marketing. Your Windows 7 OEM key will still activate Windows 10 on your XPS 8100 even if you have never installed Windows 10 before.
Note the XPS 8100 has a Legacy BIOS so use the "MBR partition scheme for BIOS" when making a Bootable USB.
For more details see here:
Its a bit cumbersome to Download Windows 7 and update the installation media. I hence recommend going to Windows 10. If you want the Windows 7 .iso there is a means of Downloading it from Microsoft. See here for details:
Yeah, I ran diagnostics at the beginning of all this, and it came back with the hard drive error code. I guess I was hoping that I could still run Windows startup repair against it, in case it was a minor issue, since the drive is still listed in the BIOS so it's not completely dead. However, since it looks like booting from the install DVD and a USB drive are both just ending up in a black screen (which is different from the blue partly loaded repair startup screen I got when booting from the HD, but still stuck either way)...I guess that's not going to work. I didn't realize that a bad hard drive could prevent Windows from loading from a disk, but googling around indicates that can happen. Soo.....yeah, probably not much else I can do at this point without a second, new hard drive.
Startup repair DOES NOT FIX physically damaged drives. in general I cannot think of any instance where the "repair" actually worked EVER even with a working non damaged drive.
Just to update, I got a new HDD and installed it. Windows still didn't boot up from USB or DVD drive - it would start, and then go to a black screen with no cursor. So, I disconnected the power/data cables from the original bad drive and retried. This time, Windows installation screens came up and I was able to install Windows 7 on the new drive (working around one issue where I needed to download/install drivers for the Win install to complete successfully - it initially said it couldn't install Windows on my hardware). Once Windows was installed, I installed some updates to get to SP1, and then turned off the PC and tried plugging in the power/data cables to the original HDD again. This time, when I restarted (specifying that the new HDD should be booted), Windows came up. I couldn't access the original HDD because of an error - Windows said it needed to be formatted. So I restarted into safe mode command prompt and then ran chkdsk /f on the original HDD. I had to run it twice - I still couldn't access it after the first run. After the second run, however, I was able to see files on it again. I then rebooted and tried specifying the original HDD for booting. This time it did come up and successfully ran Windows startup repair, though it couldn't find anything to fix (presumably because chkdsk already fixed it?).
So...I think I'm past the worst of it, hopefully, and just need to clone everything to the new hard drive now. So apparently, the problems I was having with booting Windows from a DVD/USB were due to a bad disk being connected, screwing up Windows' startup attempts.