Deploy a project update without uninstalling previous version

I'm using Visual Studio 2008 to develop a C# project, which is deployed via .msi distributed on a CD or via ftp transfer. I need to be able to upload or send out the latest version, and have it update the program without requiring the user to uninstall the old copy through Add\Remove programs. I'm sure this is probably a setting to set in the project deployment wizard, but I'm a relative newbie to deploying things through VS 2005\2008, so any information you can give me would help immensely!

Thanks!
Joe