Offices with windows are distributed according to years of service, not according to seniority or position on the management ladder. When group moves, it receives a block of offices which usually consists of approximately 50/50 mix of window and inner offices. 50/50 is not a rule, it is just how Microsoft buildings are constructed [:)]. List of team members is sorted by the years of service and window offices are dirstributed according to the list.
Groups with higher number of senior developers may receive a bit more window offices. Our team is fairly senior. A few weeks ago we celebrated our development manager's 15 years anniversary. At least three more developers in the team are past 10 years mark. So we have maybe 70% window offices and 30% inner ones. I got my first window office after approximately 5 years at Microsoft (this year is my 8th year here).
Typically each developer has individual office with a door. If group grows, it may eventualy run out of offices. Until group moves to a new location, some devs may get doubled, i.e. two people may be sitting in the same room. Doubling typically happens in the reverse order, but seniority in this case is taken into account, so years of services play smaller role.
Since senoirity and management position typically correlates with how long person worked for the company, managers and architects usually sit in window offices. However, I know a few who don't. My guess is that they probably joined Microsoft at the senior position, but haven't worked long enough to get to the window office. I think if someone joins Microsoft at VP position, then he or she might get a window office immediately, but it is generally an exception to the rule.
If someone who counted enough years moves to another team and the team does not have window office available, newcomer might have to wait until team moves or more window offices become available in the same building/floor. Typically people are not getting bumped out of the window office simply because someone more senior joins the team. However, during the move sometimes people do lose the window.