Problems with Altera Quarus 5.1 on Vista

Are you using version 5.1 because you are updating an old design that was done with that version? If you don't specifically need to use the old version, you will probably be better off using the latest version. Many major feature additions, enhancements to old features, bug fixes, etc. have been made to Quartus between versions 5.1 and the current version 7.1. Version 7.1 is available on the Altera web site for download of either the subscription edition or free web edition.