Follow these steps:
1.Open the affected drawing
2.Type "dxfout" in the command prompt
3.Save the dxf in the location you can easily access
4.Now, type "dxfin" in the command prompt
5.Browse the file you save in step 3
6.Save the file as dwg
The Produced by an Autodesk Educational Product should be removed upon completion of the steps above. If the educational product plot stamp still appears, check the xref file and apply the steps I’ve mentioned above.
this information i too got from another forum if u want to check the answers related to this anymore goto autocad help in the help select search and type the question u will get.....
i hope this information helps u