SQL Database "Does not exist or access denied"

Help...I have studied C++ for a year and now my company says I need to be the forerunner in a team to design a web application that uses Visual Basic. I have no idea what I'm doing. I have gotten far enough for it to try to connect to a database, but I get that error message. I know the database exists on this machine, it is running SQL Server 2000. I know that I have rights to the database because I can log onto it through Enterprise Manager. What else could be causing this error? Thanks, Jim