What is this error about in sql server? Trying to restore database, getting a dialog in SQL Server Management Studio 


  You cannot select
  backup sets from different databases"


Any idea why I am getting this error?

BTW backup is from SQL Server 2000 and trying to restore in SQL Server 2000 using SQL Server 2008 R2 Management Studio
The message usually means that you're trying to restore a backup of another - unrelated - database over your existing database. Double check that the backup you're restoring is really of the database you think it is.

Edit: If I remember correctly, you can also get this message if you're trying to restore a differential backup without first restoring the correct full backup.