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...
.docx or .xlsx or .pptx = A Word, Excel or PowerPoint 2007 file - Click here to Download Microsoft Compatibility Pack
.pdf = Adobe Acrobat - Click here to Download Acrobat Reader
.xls = Microsoft Excel - Click here to Download Excel Viewer
.ppt or .pps = Microsoft Powerpoint - Click here to Download Powerpoint Viewer