MySQL Forums

I've renamed this post as I now have the solution, and to save other people two days trying to find the answer.

We were running MySQL 5.0 under the 32 bit Windows Server 2003 and using the 3.51 ODBC Connector, with the connection string
"DRIVER={MySQL ODBC 5.1 Driver};SERVER=localhost;DATABASE=dbname;UID=userid;PWD=password;"

We then migrated to the 64 bit version of MySQL 5.1 under Windows 2003 Server 64 bit, together with 64 bit IIS and 64 bit 5.1 ODBC Connector.

ASP was returning "Provider not found" messages. To get the application to work, we needed to install a 64bit version of MSDASQL by downloading a hotfix (WindowsServer2003.WindowsXP-KB948459-v2-x64-ENU.exe) from Microsoft at:
http://www.microsoft.com/downloads/details.aspx?FamilyID=000364db-5e8b-44a8-b9be-ca44d18b059b&displaylang=en

This installs a 64bit version of MSDASQL. However, it still doesn't seem to be the "Default Provider" so the connection string now needs to be:
"Provider=MSDASQL;DRIVER={MySQL ODBC 5.1 Driver};SERVER=localhost;DATABASE=dbname;UID=userid;PWD=password;"

and that worked, hoorah! and we successfully upgraded our hosted CRM system www.reallysimplesystems.com over the weekend.

I'm suprised that nobody else has documented this, but hopefully Google will find this post for those who need it.