I'm wondering if this is a cache/temporary internet files issue.
I've had this happen before. You're download a file, the connection quits before it's done.. the file saves to your disk.. It's "corrupt". Now every time you download the same file, you get the same file size, and it's always "corrupt". The reason for this is that you're not actually redownloading the file, you're getting the file back from the "temporary internet files" directory.
Deleting all offline files through the tools -> internet options menu can solve this, if it's actually the issue.
If when you re-download the files, it actually goes through the download process and isn't near instantaneous, this probably isn't the issue.
--NipsMG