eBooks can use different file formats, with each requiring different software to access its contents. So, without knowing the relevant format, nobody here will be able provide you with a definitive answer as to the way forward. However I'll still have a go at pointing you in the right direction:
Return to your email and click on the file attachment. Do NOT choose the option to 'open' it. Instead choose the option to 'save' it. Depending upon how your web browser (or email client) is configured, you'll either be asked to choose a location to save the file to or it will automatically be saved into your default folder (which is probably 'Downloads').
When you've saved the file, go to its location. RIGHT-click on the file and select 'Properties'. (The 'General' tab will probably be pre-selected. If it's not, click on it).
You'll see a line labelled 'Type of file' and another labelled 'Opens with'. The first line includes the file extension (e.g. .pdf, .epub, or .azw) and the second one shows you the program that Windows thinks should be used to open that type of file. (Your post suggests that it will be Windows Media Center, or a component of it, even though that's clearly wrong).
If the file extension shown is one that you're familiar with, and you know which program ought to be used to open it, click on 'Change' in order to assign the correct program to that file type. (e.g. if the file type is .pdf and you normally use Adobe Reader to open PDF files, click on 'Change' and select 'Adobe Reader'. Then double-clicking on the file should open it correctly).
If the file type is one which leaves you mystified as to what should be used to open it, ask us for advice about which program you need. (e.g. if the file type turns out to be .azw then the eBook was intended to be read on a Kindle and you'll ned to download a program called Kindle for PC in order to be able to read it).