Original windows and external doors are usually common property — meaning the owners corporation pays. Internal doors and any windows you've installed or replaced yourself (with approval) are usually your responsibility.
This varies between states and individual strata plans. NSW treats original windows as common property by default; some older schemes specifically transfer responsibility to lot owners by by-law. Check your by-laws and the strata plan before paying for window repairs out of pocket.
Compare written fee proposals from strata managers covering your building — free for owners corporations, no obligation.