Have you seen the Spybot S&D write up about that message? Here's a quote:
During update, I get an error message the update is forbidden / bad checksum!!!
Some user have reported they are not able to download updates. The 'Search for updates' reveals updates, but they cannot be downloaded. Often a 'HTTP Error 403' or 'bad checksum' is displayed. This has a simple reason - thousands of people trying to download from the default server, which can't handle such a burden. If you try again, have a second look at the menu bar. It has a pull-down item to select a mirror. Click the arrow beside it, and select a different location, where you'll most probably have better chances to download. (Spybot-S&D 1.3 will set a random mirror as the default to better distribute the load)
Current problems with the main installer download have the same reason - the servers not being able to handle such large amounts of visitors. Even the University isn't able to deal with many thousands of downloads at the same time. Please have some patience; more mirrors are being added again.
Usually it's a problem with the Spybot download server being busy. Try selecting a different download server from the selections at the top of the window.
Hope this helps.
While attempting to download the latest beta includes from S & D Spybot, the download is aborted and I'm told !!!Bad Checksum!, exclamation points are S&D's. I believe I know what checksum refers to,anyone else with similar or idea why? Solution? etc.