Modify the download URL to this and check if you are able to download.
webClient.DownloadFileAsync(new Uri("http://download-codeplex.sec.s-msft.com/Download/Release?ProjectName=dotnetnuke&DownloadId=815672&FileTime=130397249034130000&Build=20885"), @"C:\DNN_Platform_07.02.02_Install.zip");
That is the actual URL obtained by inspecting the site with Firebug, when the download request is fired off from the previous URL. While this works for the current release, I'm not really sure if this will fetch the latest builds over time. I suspect that the FileTime will change.