Andrewefaw :
I went to this site (http://windows.microsoft.com/en-us/windows-8/upgrade-product-key-only) and clicked the "Install Windows 8" icon, and it downloaded a file that was about 5 megabytes a little more. I am building a custom pc in a few weeks, so if I stick this on a USB drive, will it install Windows, or do I need something else to use as an ISO?
What you downloaded is only the windows 8 installer not windows.
Run that little file, to start the process of downloading windows (8 or 8.1) after it finished downloading Windows (a little more that 2GB), you'll have the option to install Windows now, later, or using media, USB or with an ISO file to burn it to a DVD. You can not use the the ISO file from a USB drive.
Read carefully from that page:
Upgrade Windows with only a product key
If you bought a Windows 8.1 or Windows 8 product key, you can download and install Windows from this page.
You'll have the option to install Windows now, later, or using media with an ISO file.
Before you begin:
• Make sure you’re on the PC you want to upgrade. If you're updating to Windows 8.1,
this PC must be running Windows 7 or Windows 8.1 Preview. If you already have the latest version of Windows 8 installed on your PC, you can update to Windows 8.1 in the Windows Store for free.
If you're upgrading to Windows 8, this PC must be running Windows 7, Windows Vista, Windows XP with Service Pack 3 (SP3), Windows 8 Release Preview, Windows 8 Consumer Preview, or Windows Developer Preview.
Windows Setup checks that your PC meets all system requirements before it installs Windows 8.1 or Windows 8.
• Have your product key handy. You can find it in your online order info, in the confirmation email for your purchase, or on the packaging. The product key will look something like this:
XXXXX- XXXXX- XXXXX- XXXXX- XXXXX.
• Make a backup of everything you want to keep. Save it somewhere you can get to it later, like an external drive or a DVD.
Warning
Don't format your hard drive before installing Windows. Your current version of Windows must be running when you begin installing Windows 8.1 or Windows 8.
When you’re ready, tap or click Install Windows 8.1 or Install Windows 8:
Install Windows 8.1
Install Windows 8