I don't know what it is and why it happens. Could be many different things. Do you see the same corruption on the screen, inside HyperSnap window, or does it only appear when you load the saved file? If only in the file, could indicate a bug in saving code, or a corrupted hard disk space, anything in between. If you see this immediatelly after the capture on the screen, could be a bug in the capture code, or in the graphics card driver, or even damaged graphics card. Or I don't know what else.
First thing I would advise, upgrade HyperSnap to the latest version 7, currently 7.29.10, from
http://www.hyperionics.com/?Page=hsdx/downloads.asp page. Your license should be valid for any version 7.xx. See if using the newer version changes anything.
Greg