using PowerPoint = Microsoft.Office.Interop.PowerPoint;
PowerPoint.Application powerpnt;
PowerPoint.Presentations objpresentation;
string powerpointfilename;
powerpointfilename= @"mypowerpoint.ppt";
powerpnt= new PowerPoint.ApplicationClass();
powerpnt.Visible = Microsoft.Office.Core.MsoTriState.msoTrue;
objpresentation= powerpnt.Presentations ;
objpresentation.Open ( powerpointfilename,MsoTriState.msoFalse ,
MsoTriState.msoTrue ,MsoTriState.msoTrue );
check if its helps