Your files don't seem to be corrupted. You just need the correct codec installed to enable them to be viewed.

I've never been able to get the Microsoft Canon Raw Codec pack to enable previewing .cr2 files from my 50D or 7D using the Microsoft photo viewer. I'm running Windows 7 on all three of my main computers. I must admit I haven't spent a lot of time on it when there are simple alternatives that work flawlessly. Here are two that I use most often:

• Irfanview is a free (for personal use) program that is easy to install, uses very few resources, and opens raw files extremely fast. In addition to the main program you'll also need to install the PlugIns (current version 4.40) and the Canon DLLs. Click the "download" and "plugIns" tab on the main page to see a list of available download servers. You can also install "Irfanview Shell Extension" (at the "PlugIns" tab) to add some Irfanview functions to the Windows Context Menu (right click menu). I currently have my file associations for .cr2 files set to open using Irfanview. If I want to preview an image I double click the file and it opens almost instantly. I can then use the right and left arrows to navigate through all of the images in the same folder.

• Digital Photo Professional is the editing program provided by Canon for use with their digital EOS cameras. If you no longer have the discs that came with your cameras, you can still download a full version via the Canon support page for your camera. Click on the "Software" dropdown and download the EOS Solution Disk Software. This version contains DPP 3. Although there is a much improved version 4 of DPP, it does not support older models such as the 50D.
  DPP4 does support your 70D and 5DII, and I would encourage you to try it with images from those cameras. I prefer it to Lightroom for much of my raw editing. You can have both versions 3 and 4 installed at the same time without any issues. All of the edits I did to 5DII, 5DIII, and 7D images in version 3 before the introduction of version 4 are not applied when those images are opened in version 4, so I keep it installed for when I need to go back and access a raw file I edited prior to switching over to version 4.