We've had some reports of people have "file corrupted" messages from some of the videos they download from www.microsoft.com/uk/launch2007/dev. We're looking into this, looks like the download is interrupted which causes the problem ie it doesn't download the whole file.
If you encounter this you can get around it in the short term by watching the streaming version, but we're working to fix it asap. I'll post something here when we have it fixed. Many apologies if this is causing you pain.