Mod parent up. Voline is absolutely right. People think that firms decide how much their price will "sell" at. That's only half of the equation. Now, Voline, since there really are no substitutions for Microsoft's product, the fact that Microsoft has all but eliminated consumer surplus by offering 200 different SKU's for their OS and charging this demand determined price, the price increase that would make up for the corporate tax has already happened in the past. At this point, any increase in tax is really a hit on their profit margins.
This gets me to thinking, if MS's software is as essential to business as oil is, does it share its inelasticity? Part of the problem is that Microsoft isn't always selling to consumers, but to a manufacturer's input in the form of OEM installs. That manufacturer's output is definitely substitutable and shows a high degree of elasticity, which is probably why Dell is trying out Ubuntu installs in some of its laptops.