On importance of advertisements.

http://blogs.eweek.com/applewatch/content/macbook/microsoft_ceo_scoffs_at_mac_share_gains.html

I kind of agree with Ballmer on several points here.
In particular, on importance of advertisements. I think it is fair to say that ads are much less effective for microsoft, for several reasons.

Firstly, target audience size:
Suppose, Microsoft has marketshare 90%
Suppose Microsoft makes extremely powerful but expensive advertisement campaign which converts 5% of remaining non-windows users (whom are 10%) to windows. That'll raise Microsoft's marketshare from 90% to 90.5%

Suppose Apple has marketshare 5% and they make such campaign which converts 5% of those 95% non-mac users to mac. That'll raise apple's marketshare from 5% to 9.75% . Both relative and absolute increases are much higher for Apple than for Microsoft.

Secondly, visibility. It is fair to say that those 10% of non-windows users are aware of windows; ads don't have effect of increasing the awareness.

Thirdly, Microsoft, unlike Apple, is not relying on selling highly overpriced hardware or software. Many mac users own Microsoft Windows.
You say you had to buy windows to run software that is not available for mac? Blame apple for not providing cross platform development tools, and for licensing SDK in such a way that adaptations to other build environments cannot be redistributed.