Especially in the realm of antivirus software, there are certain APIs (like on the kernel level) that work perfectly fine in one OS but then are depreciated, heavily modified, or even removed in the next one.
for the record I have tried 2009 and it installs and works fine that did not break anything so why would 2007. its just software.
It's been, what, 5 years since 2007, and two new versions of Windows have been launched in that time (which is enough time for a feature to be depreciated and then removed). It's almost like asking why I can't run Windows Internet Explorer 10 on OSX; from the software's point of view it might as well be running on a ternary system
It's just that some (many) of the people here feel it's a waste of time to try installing an unsupported piece of software on an OS that's been pretty much completely reworked since its release, especially if you have easy access to a supported version of the software. Is there a reason you want to do this, or just for curiosity?
and since you requested some kind of guidance, a quick search brought up: http://www.microsoft...07-t165466.html