Just edit App_Data\FileVista.config and set the AuthenticationKey property to the same value as in your initial installation's config:
<?xml version="1.0" encoding="utf-8"?>
<FileVistaConfiguration>
<DbProvider>System.Data.VistaDB</DbProvider>
<DbConnectionString>Data Source=|DataDirectory|FileVista.vdb5; Open Mode=NonexclusiveReadWrite</DbConnectionString>
<DbSchemaVersion>7.3.6</DbSchemaVersion>
<AuthenticationKey>3102FF196003A0B05F9B812376A044786E803D4F9BF5795F2819CE5253776A82</AuthenticationKey>
<RecoveryModeEnabled>false</RecoveryModeEnabled>
</FileVistaConfiguration>