If you are downloading these ISO files via a Windows computer, you may want to use an excellent program called ImgBurn to burn your downloaded ISO file to the CD as an image, which is necessary to make the CD into an install disk. ImgBurn is one of the very best free utilities (CD/DVD/Blu-ray burner) for Windows computers. You can check it out here, and download it here. It is an easy to use but full-featured lightweight burner for CDs, DVDs, HD DVDs, and Blu-ray disks. In addition to the normal tasks that you would expect from a burner program, it can read a disk to an image file, create an image file from files on your computer or network, and check image files written to disks for accuracy. For the making of the Ubuntu install disk (also called a "live CD"), you burn the ISO file as an image to your CD. I recommend that you burn it at a slower speed such as around 16X for a higher quality data burn and less risk of any possible read errors when you install it on your system. You may also use a program called Infra Recorder, at this link.
For Advanced Users
I recommend that you do an md5sum check on your ISO file on your hard disk before you burn it as an image to your CD, to be sure that it is accurate to the last data bit. You can get the free Windows program called ChaosMDS program at this link, which is a free program that will produce the MD5 hash for any file, including the Ubuntu ISO install file. You can get the official MD5 hash string from here. If the hash result from the ChaosMDS program agrees exactly with the official MD5 hash string, the probability is nearly 100% that the file was downloaded perfectly.
A note for your use but not required to make an install disk: To copy a disk (CD/DVD etc.) using ImgBurn when you have only a single CD/DVD drive in your computer, you must copy the image of the CD or DVD to your hard drive, place a new CD/DVD in your CD/DVD drive, then burn the image of the CD/DVD that you saved to your hard drive to the new CD/DVD.
As a Side Note
After you install Ubuntu Linux, you may find it interesting that Linux can run many Windows programs via a Linux program called "Wine." I suggest that you also install the ImgBurn disk burner within Wine. (After you first install Ubuntu Linux on your computer, install Wine first from the available programs in Ubuntu. From the Ubuntu menu, use Applications → Ubuntu Software Center). It is probably best if you use the Windows XP simulation. Before you install ImgBurn, configure Wine and make it autodetect all drives that it will autodetect. Also, before it will produce any sounds, you have to go to that part of the configuration and click apply.
To install ImgBurn, copy the downloaded install file to your Downloads directory (if it is not already there). To get to the Downloads folder (directory), from the Ubuntu menu, select Places → Downloads.
To install the ImgBurn install file, go to the Downloads folder, right click on it and open it with Wine Windows Program Loader. If it gives an error message saying that it does not have the required permission to run the install program, right click on it and choose Properties. Then choose the Permissions tab. Check "Allow Executing File as Program."
After it is installed, if ImgBurn doesn't see your CD/DVD drive when it comes time to write to that drive, do this: Go to Tools → Settings → I/O and choose the option within the Interface menu "ASPI – WNASPI32.DLL".
Your drive(s) should be found. There will probably be no need to “autodetect” again or specify drives in the wine configuration. ImgBurn should work beautifully.
If you wish to get the very latest stable version of Wine (but possibly a bit less stable than the default version offered in the Ubuntu Software Center), perform the following using the Ubuntu menu:
1. System→Administration→Synaptic Package Manager,
2. Enter your system password,
3. Settings→Repositories,
4. Other Software,
5. Add.
6. Then in the APT line type ppa:ubuntu-wine/ppa
7. Click on the Add Source button,
8. Close,
9. Close,
10. Reload. (Note that if an error in signature verification occurs, a later version of wine than that which is in the Ubuntu repositories may not be implemented.)
The newer version of Wine is likely to run a greater number of Windows programs. However, obtaining a newer version of Wine will probably be completely unnecessary for running ImgBurn.