In bin folder there is CrystalDecisions.Web.dll delete that file and try.
Or
Look at the Register Assembly directive on the page where your CrystalReportsViewer is. My guess is that your directive is calling for the version that is native to VS2005. You can either remove the CrystalReportsViewer and the directive and then add the CrystalReportsView back in via VS2008 and it will insert the approprate directive back into your page, or you could just make sure the page is only calling for the VS2008 assembly.
<%@ Register Assembly="CrystalDecisions.Web, Version=10.5.3700.0, Culture=neutral, PublicKeyToken=692fbea5521e1304"
Namespace="CrystalDecisions.Web" TagPrefix="CR" %>