To use NAntAddin 2005 with Visual Studio 2008 download the latest binaries here.
Extract the NAntAddIn files to My Documents\Visual Studio 2008\Addins
(note: you may need to create the Addins folder if it doesn't exist).



Open the NAntAddin.AddIn file in a text editor and change the Version node from 8.x to 9.0:



Close the file. Start Visual Studio 2008 and open the Add-in Manager (Tools -> Add-in Manager).



NAntAddin should be in the list. Check the Available and Startup checkboxes.
The NAntAddin should now be available from the Tools menu.



The last bit of configuration needed is to tell NAntAddin where the NAnt executable is located. You can do this via the NAntAddin options panel.

 

You are now good to go with NAntAddin for Visual Studio 2008.

Links:
NAntAddin for VisualStudio 2005 Download

From the installed NAnt folder copy the nant.xsd file to C:\Program Files\Microsoft Visual Studio 9.0\Xml\Schemas

In Visual Studio 2008, create a new XML file, add the root node <project>.
When you add the xmlns attribute to the project node the nant schema will now be available in the drop-down list.

NAnt intellisense and schema validation will now be available for the XML document.



Links:
NAnt website