Un-install the dll from GAC before building the solution.
“C:\Program Files\Microsoft Visual Studio 8\SDK\v2.0\Bin\gacutil.exe” /u $(TargetName)
http://geekswithblogs.net/sthomas/archive/2006/08/28/89593.aspx
“C:\Program Files\Microsoft Visual Studio 8\SDK\v2.0\Bin\gacutil.exe” /i $(TargetPath)