(2-part post):
This problem is most likely to arise because you've been sent an attachment which doesn't have a suitable program for viewing it on your computer. (You can't associate a file type with a program if no suitable program exists!)
For example, if someone sends you information on an Excel spreadsheet you won't be able to open it unless you've got Excel (or a compatible program) on your PC. So, if the files you're getting have a .xls extension, you need to get hold of something like Microsoft's free Excel Viewer software. When you install it, it will automatically create the correct file association:
http://www.microsoft.com/downloads/details.asp x?********=c8378bf4-996c-4569-b547-75edbd03aaf 0&displaylang=EN
Similarly, if the files you're receiving have a .pub extension, you'll need Microsoft Publisher (or a compatible program) to view them. Microsoft doesn't provide a 'viewer' program, as such, for Publisher but you can download a free trial of Publisher (which carries on working as a 'viewer' after the trial period expires):
http://office.microsoft.com/en-us/publisher/HA 102339061033.aspx
If the files you're receiving have a different extension, and you've not got a clue what to open them with, post again (telling us what the file extension is) and we'll direct you to suitable software.