Problem opening some ( a lot) of attachments.....error message

I get this message " You are attempting to open a file of type "(.dat)   These files are used by the operating system and by various programs. Editing or modifying them could damage your system.  If you still want to open the file click open with, otherwise click cancel.

So, of course I want to see this attachment and I click open with and then I get this error message : Windows cannot open this file : winmail,dat  To open this file Windows needs to know what program created it  Windows can go online to look it up automatically or you can manually select it from a list of programs on your computer

So then I let windows try to look this up online and it brings me to a general website that sells programs.  It never tells me which program I need ?  A few people have done some things on my computer trying to clean it up a bit and I was wondering if maybe they could have deleted a program I should have kept ?  Because this wasn't happening in the beginning, just lately.  Any ideas would be greatly appreciated.

Thanks

Susan