A while ago I posted a walkthrough on how to call .NET code from VBA. This code was hacked up a long time ago using VS2003 at a time when I maybe didn't have the required knowledge to adequately demonstrate the process. I have disabled this post as some people, with VS2005, are having problems with it.
Instead here is a really good and free, we all like free stuff, ebook on COM and .NET Interoperability.
http://www.apress.com/free/index.html
You will find the book about half way down the page. This book is completely worth the download and is filled with all the information you'll need to bridge the gap between COM and .NET.