2011 Word for Mac problems

When I open a file in my Word for Mac program that was created in Word and is on a portable drive it opens fine. If I first save the file to the Mac desktop and then try to open I get the message " This is not a valid file name". This problem is on my IMac but it doesn't happen on my Macbook . Both are running Lion and both have the sane version of Word for Mac. (2011). Has anyone run into this problem and did you resolve it?