Well not everyone has updated to Vista. (And some never will. ) I'm not sure if Delphi will compile for XP when on a Vista system, will it?

Lazarus will compile to exactly whatever system to tell it to. So long as you set it and it's supported. There is some issues compiling to Mac or Linux from Windows. But the other way around is quite easy I'm told. Laz is a VERY good solution if you wanted to go cross-platform. I'd recommend it.