Hello billwoodard11,
Thanks for the question. If your computer is not compatible with HD 1080p content, you should receive a prompt stating so - but should still be able to download the content for use on other devices. For more information, check out this resource:
Purchasing and viewing HD videos in iTunes - Apple Support
http://support.apple.com/en-us/HT201850
How to purchase, download, or view HD 1080p content
iTunes 10.6 by default is set to purchase, download, and playback HD content in 720p. You must change your local preference setting if you prefer to download, purchase, and playback HD content in 1080p. Depending on your computer's hardware configurations, you may be unable to purchase, rent, or playback content in 1080p. iTunes may display the following dialog:

On Mac or PC
- Download the latest version of iTunes. You can download iTunes for free.
- From the iTunes menu, choose iTunes > Preferences > Store (for PC, choose Edit > Preferences > Store).
- Next to "When downloading High Definition videos, prefer:" set your preference to 1080p1.
- Click OK. Now, when you purchase or download HD content, your computer will always download in 1080p, when available.
Depending on your computer's hardware limitations, you may be unable to purchase, rent, or view HD content from the iTunes Store. If you are trying to view HD video on an external display, the display must have a digital connection (DVI, DisplayPort, or HDMI) and support HDCP.
Check out the following resource for information on downloading past purchases, including information on playing purchases on Apple TV:
Download past purchases - Apple Support
http://support.apple.com/en-us/HT2519
Thanks,
Matt M.