There is no such thing as "Windows XP software" and "Windows Vista software." Rather, software can be stated to run under Windows XP, Windows Vista, and/or other operating systems. For instance, Microsoft Office 2007 will run under Windows 2000, Windows XP, Windows Vista, and Windows 7.
That said, almost all software that runs under Windows Vista will run under Windows 7, and most software that runs under Windows XP can be run under both Windows Vista and Windows 7. Conflict only exists with specific applications that were either poorly designed or designed only for the one specific operating system.
Regardless, an application for Mac OS X cannot run under Windows--they are entirely different operating systems that run entirely different applications.
Hope this helps,
So can 7 make vista and xp software work together without conflicting? or you work with MAC software on the 7 system as well? I just heard of this, don't know if it's true.