I have a ASP.NET 2.0 web application that should upload a ppt file and then extract its slides to images. For that I have imported office.dll and Microsoft.Office.Interop.Po
Don't ever use Office Interop from an ASP.NET application. See Considerations for server-side Automation of Office.
However, if you have no alternative, on Windows Server 2008 you need to create the following directories to get this to work:
Windows 2008 Server x64: C:\Windows\SysWOW64\config\systemprofile\Desktop Windows 2008 Server x86: C:\Windows\System32\config\systemprofile\Desktop