Could you provide some details about your computer/OS?
Also see the following document to troubleshoot the download experience: http://helpx.adobe.com/creative-suite/kb/troubleshoot-download-assista nt.html
Invalid checksum error is thrown if the file download was corrupt. It coudl happen due to slow/spotty internet connections. If the corruption is minor you mayt be able to work around it by switching off file verification.