If you are going to use DNP on 64-bit version of Windows and manage 32-bit applications at the same time (e.g. mail server) you need to switch ASP.NET 2.0 in IIS to 32-bit mode.
First of all using adsutil.vbs utility change Enable32BitAppOnWin64 setting in IIS metabase. adsutil.vbs script is usually located in c:\Inetpub\AdminScripts folder, so you should perform CD command first:
Start -> Run... -> cmd -> [ENTER]
C> cd c:\Inetpub\AdminScripts
then run the following command to enable 32-bit IIS ISAPI module:
cscript.exe adsutil.vbs set W3SVC/AppPools/Enable32BitAppOnWin64 "1"
Then update ASP.NET 2.0 ISAPI registrations in IIS metabase:
Make sure that the status of ASP.NET version 2.0.50727 (32-bit) is set to Allowed in the Web service extension list in Internet Information Services Manager.