If you get an email and when you try to open the attachment Windows does not know what program to open it with - Then it is likely you don't have the program needed.

How do I know what program I need?
Look at the file extension on the name of the file (the three characters after the last . (dot)

Here are some common ones with links to free downloads.

If your file ends with...

.pdf = Adobe Acrobat  - Click here to Download Acrobat Reader

.xls = Microsoft Excel - Click here to Download Excel Viewer

.ppt = Microsoft Powerpoint - Click here to Download Powerpoint Viewer

.zip = WinZip - Click here to Download WinZip