garbage collection vs arc

do i understand this correctly,  arc(automatic reference counting) in ios objective-c is better than garbage collection because it is at compile time not at runtime that slows down apps.  and a)arc is for phones only else arc would be used instead of garbage collection on personal computers as well, so for instance mac os X lion uses garbage collection and windows for pcs uses garbage collection while iphones use arc and microsoft phones use arc? if microsoft phones don't use arc, why done they?

b)why doesn't microsoft just use arc instead of garbage collection in general.  have the compiler scan through the code and insert release statements where necessary.  this probably isn't possible right?  why is it possible on the ios phone.  is microsoft lacking?