I am developing an Excel 2003 add-in using Visual Studio 2008. My add-in loads fine; however, it loads plug-ins from other C# DLL's. I would like this to be done dynamically at run time so referencing them during development is something I would rather not do. Anyways, anytime I try to load a DLL from the Excel add-in at start up, it throws a security exception. This particular example is HTML Agility Pack. It's not a plug-in but a plug-in's dependency. But nonetheless it won't even load:

{System.IO.FileLoadException: Could not load file or assembly 'HtmlAgilityPack, Version=1.4.0.0, Culture=neutral, PublicKeyToken=bd319b19eaf3b43a' or one of its dependencies. Failed to grant permission to execute. (Exception from HRESULT: 0x80131418)
File name: 'HtmlAgilityPack, Version=1.4.0.0, Culture=neutral, PublicKeyToken=bd319b19eaf3b43a' ---> System.Security.Policy.PolicyException: Execution permission cannot be acquired.
   at System.Security.SecurityManager.ResolvePolicy(Evidence evidence, PermissionSet reqdPset, PermissionSet optPset, PermissionSet denyPset, PermissionSet& denied, Boolean checkExecutionPermission)
   at System.Security.SecurityManager.ResolvePolicy(Evidence evidence, PermissionSet reqdPset, PermissionSet optPset, PermissionSet denyPset, PermissionSet& denied, Int32& securitySpecialFlags, Boolean checkExecutionPermission)
   at System.Reflection.Assembly.nLoadFile(String path, Evidence evidence)
   at System.Reflection.Assembly.LoadFile(String path)
   at Cjack.Druid.SourcePluginManager.LoadPlugin(String filePath) in C:\Documents and Settings\Annie Tormey\My Documents\Visual Studio 2008\Projects\DruidAddin2003\Druid\SourcePluginManager.cs:line 26
}

This is extremely frustrating because it runs perfectly fine for Office 2010 and as a standalone application. Thank-you to anyone who can give me an answer as to why this is happening or a solution to fix it. Thank-you for your time.