I've never had any problems with the copy of windows 7 I bought from the microsoft online store. I've had to redownload it two or three times now, and it's always been there (although the page is near impossible to find). I had a screenshot of the page, but imgur is down today so I can't link it.
Actually, apparently it's 12 months until you can no longer download the OS again:
"Be kindly informed that the availability of the software download, purchased at the Microsoft Store, is limited to one year.
Please understand that as you placed your order more than one year ago, Microsoft Store does not have access to your order and cannot reset the download link."
