I have a brand new Vista Premium system out of the box. I have been having several stability issues on the previous system (XP Home) so I upgraded to Vista Premium on a new platform. After talking with Deerfield support this morning I was advised to upgrade to 5.1. I was on 5.0.3752.0. I backed up my files and clicked on the new installer program. It told me I had to uninstall the old program first. I did this and then ran the 5.1 installer again. It gets about 99% complete and then says there is a windows installer problem and to contact my installer provider. It then rolls back the installation. Now I have no phone system. I then tried reinstalling the old version, reapplied the backups and all is back to normal. Apparently 5.1 does not work on Vista Premium??