Before you can upgrade a WSS 2.0 database to WSS 3.0 you have to run PRESCAN.EXE against it. This tool is part of WSS 3.0 - with other words you have to install WSS 3.0 get hands on this tool.
Many customers asked to provide this tool as a separate download to allow them to run this tool before they install WSS 3.0.
Here is now the download location for this tool:http://www.microsoft.com/downloads/details.aspx?FamilyId=E8A00B1F-6F45-42CD-8E56-E62C20FEB2F1&displaylang=en
Stefan Gossner posts about running PRESCAN.EXE against a Windows SharePoint Services (WSS) 2.0 database