that doc file may be damaged...so you need to try and save it now....
How to troubleshoot error messages that you receive in Word 2003 or in Word 2002 (Part 1)
Does the problem only occur with a particular document (or collection of documents)?
if you are able to successfully open it, then I suggest copying it's contents into a new document...but do not make a copy of the file. I hope that's not confusing right?
but it could just mean you have not updated your app....get the service pack if you have not done so already. I know I had to put up with this "converter" message because the last place I worked at, the tech guy never bothered to update my office apps. Different versions of Office all over the place.