You're off topic for this forum, which is dedicated to Windows Live OneCare.
You should probably contact the maker of the software to confirm your system requirements. Windows Vista is based on the NT kernel, so their installer is apparently detecting the Windows version and throwing that error due to incompatibility.
You may also want to try the Microsoft Public newsgroups here: http://www.microsoft.com/communities/newsgroups/list/en-us/default.aspx
-steve
Microsoft MVP Windows Live / Windows Live OneCare, Live Mesh, & MS Security Essentials Forums Moderator