Microsoft is probably aggressive about the windows trademark as it is a very generic trademark and could easily be revoked if not defended vigorously.
Even in relation to operating systems it is generic - think "window manager". Because of this it is very easy to argue that Microsoft should not have been allowed to trademark such a generic term.
A large part of it is probably a CYA attitude by the legal department, too.
If they're too conservative, and legal notices are featured prominently without reason, the price is paid by Microsoft as a whole having mediocre branding, and the legal team is not accountable. But if the trademarks slip away because they've been too liberal, heads will roll.
Thus the incentives are pretty clearly stacked up for the lawyers to insist on protection that minimizes risk to a degree that hurts Microsoft as a whole.
Even in relation to operating systems it is generic - think "window manager". Because of this it is very easy to argue that Microsoft should not have been allowed to trademark such a generic term.