you can add custom action using VBScript to delete files. first, check whether the files are there, if existing, delete them. make sure where u need use this function.
Check out the help topic "RemoveFile Table". The first (key) column is anything you'd like as long as it is unique within the keys for the records of that table (InstallShield will warn you if the key is a duplicate of another record). The removal of the file(s) listed in this table must be associated with the installation or uninstallation of a component--that's what the component field is for.