If you are using MSDE in any applications that you want to have running on Windows Vista, you'll need to be aware that Microsoft are not planning to support MSDE or SQL Server 2000 running on Windows Vista.
If your application does require a database to be installed locally (that is on the Windows Vista based client computer), you can (and should) use SQL Server 2005 Express edition which is free and offers more capability than MSDE.
This does not affect access to remote SQL Server 2000 databases, nor does it make any changes in the support policy for SQL Server 2000.
If you are affected by this, you might like to read some of the information we have on getting your MSDE based application migrated to SQL Server Express. Here are some documents to help get you started.