Converting an existing web application to a class library

Question

Hello all -
Visual Studio 2008 - .NET 2.0
I have inherited two projects.  First is a web application.  Second is a windows application.  The windows application references and uses a dll which was created from the web application.  The dll has a date of 2007.  The solution/project that builds the DLL is nowhere to be found (including the 2007 version of the source code).  In the windows application the majority of the functionality comes from classes in the DLL.  I need to make changes to these classes.  The code bases that makes up the DLL has changed since 2007.  I am concerned about the side affect of creating a new instance of the DLL and using it in the windows app - interfaces may have changed, etc.
I have tried to create a new DLL based on the web project.  I am using the "create new from existing project" wizard.  When I try to build the project I get namespace errors.  It believes that everything has already been defined.
My main question at this point is how to build a current version of the DLL.  The information I found on the internet dealt with creating class library projects from scratch.  I need to understand the subtleties (sp?) of creating a class library from an existing web application.
Any comments on the situation in general are appreciated.
Thanks in advance,
 
Blake