How to open MS word document from the SharePoint 2010 using Microsoft.Office.Interop.dll

or this you must change the identity of word component in
C:\windows\System32\comexp.msc
to be interactive user instead of launching user

For open comexp.msc :
Click on comexp.msc, it will open window  then do configuration as 
"Component Services->Computers->My Computer->DCOM Config->Microsoft Word Document".  right click properties selection "Identity" tab