Changing connection string for dataset from Oledb to SqlClient

Hello,

I have an application which is built in VB6 with SQL Server 2000. I need to upgrade this application to c# .net. The pc on which I do programming has SQL Server 15 express on it. When I create datasets with wizards for SQL Server 2000 they work fine till I am connected to the server with SQL Server 2000 on it. When I try to change the datasets so I can work at home with my SQL Server express the dataset don’t allow me to do that. I have tried making two connection strings in the applications properties. But when I try to change the dataset’s connection string to the new connection string an error appears with nothing written on it. A blank error dialogue.

Thanks in advance.