That is also what I have in my Vista auggie. Windows Defender version 1.1.1505.0 is for Vista
Windows Defender v1.1.1593.0 is for XP users.
Calendar of Updates yesterday had an update for Windows Defender, WD Version 1.1.1593.0,Eng. Version 1.1.2204.0 and Definition Vrsion 1.15.2329.4.
When I updated I got the correct Eng and Definition versions but instead of getting Windows Defender version 1.1.1593.0 I still had 1.1.1505.0. When I tried to update again it said I was up to date and a statement that Windows Defender came with vista.