The dog ate my CD.
CD's aren't a perfect medium. They often become unreadable or as in many cases, simply go missing (Damn things are always running off!).
You will be pleased to know that if you ever lose your Microsoft software, as long as you can prove your ownership, they will help you get it back.
Check out Knowledge Base Article - 326246 for more info.
Note they can also help with lost/forgotten product keys too....