A "Bad checksum" error indicates that after an update file is download there was a discrepancy between the expected MD5 hash value of that file and the actual MD5 hash value of that downloaded file as received by your system. There can be several causes for this problem, it can because the download of that file timed out and never completed or that there was a problem with the transmission itself resulting in a corrupted downloaded file.
Running Spybot 1.5 or above (Spybot » Help » About)? If so, then if you get a "Bad checksum" error, click the "Back to mirror list" button and select a different download site and try the update again.
Also see:
And: