Since you already had one installation of SQL Server done before, there was a database already created. That did not get removed. So when you reinstall, its trying to create the database with the same name, hence, the error. You need to delete the old files to continue the new installation.
From the direcotry
C:\Program Files\Microsoft SQL Server\MSSQL10.MSSQLSERVER\MSSQL\DATARemove the following files
ReportServer.mdfReportServer_log.LDFReportServerTempDB.mdfReportServerTempDB_log.LDF
Try the Following link for further help.
Reporting Services Catalog Error.
For sql-server-2012 the path is:
C:\Program Files\Microsoft SQL Server\MSSQL11.MSSQL2012\MSSQL\DATA
where MSSQL2012 is the instance name and the respective file names are:
ReportServer$MSSQL2012.mdfReportServer$MSSQL2012_log.mdfReportServer$MSSQL2012TempDB.mdfReportServer$MSSQL2012TempDB_log.mdf