Error in SSMS when attempting to restore a database
From your reply to my question, then it does appear to be too much backup history. For a one off cleardown, something along these lines will do it:


USE msdb
GO
DECLARE @days DATETIME
SET @days = DATEADD(day,-30,current_timestamp)
EXEC sp_delete_backuphistory @days
GO


That example gets rid of everything over 30 days old, so you may need to change the -30 for a value appropriate for your needs. But remember that you'll then have to build up any restores for backups over 30 days old from hand (may or may not be a problem for you, but thought I'd better mention it).

Going forward you can schedule that to run via a scheduled job  to keep you small.