"Which doesn't give Microsoft the right to use tactics that are against the law."
The law is mostly ridiculous to begin with. Every business in the U.S. could optionally (at the DOJ's disgression) be found guilty of some anti-trust violation (or anti-competitive behavior in general); that is how vague the doctrine is. Anti-trust is a legal realm, where businesses only know they are guilty of breaking the law: after the fact.
Charge prices that are too low, and they can get you for unfair competition or restraint of trade. Too high, and they get you for monopoly (the idea being that if you can charge high prices and rake in "excessively high profits", you've got monopoly power), or get you for intent to monopolize. Charge prices inline with what the competition is charging, and they can get you for collusion or conspiracy to 'set' prices.
And that is just scratching the surface on what a business can be optionally found guilty of (even though the above is enough to nail any business that sells a service or product).
Some people will say:
'Well, but businesses get to defend themselves in a court of law'; right, but the government has a nearly unlimited supply of taxpayer dollars to fund actions they deem worthy, businesses of course do not have that luxury.
Here is a fun question; how many investors remember April 3, 2000? The day the U.S. Government tanked the economy. Take a look at the Nasdaq chart, or the charts on just about any tech company - they all followed Microsoft down the drain (MSFT lost something like 15% that day). Greenspan had of course tightened rates a little too much (the government tanked it on more than one front), but this pushed the market off the cliff. This extreme economic slowdown was driven by the market crash (yes, it should have deflated, but not 3,000 points), which naturally impacted every sector of the economy; the market crash (not just deflation, but crash) was initiated by the DOJ's declaration that they would breakup Microsoft.
Don't think so? Watch the Nasdaq rally forward from here (after hanging around 2000 for the last couple months).
"You say that now, but wait until your first "computing bill" arrives from Microsoft
'We find you are behind in your payments. Please pay in full in 15 days or your computer will cease to function.'"
Microsoft has a right to control what happens to their property, and what conditions they decide to set on the sale of their property (Windows etc.). It is in Microsoft's best interest to charge prices that are reasonable to the consumer; if Microsoft pushed the price of Windows XP personal edition up to $1,250 per copy, the capital markets would create new competition over night, because it would mean there were huge sums of money to be made by undercutting what Microsoft is charging (and at $1,250 it wouldn't be difficult to underdcut). However, Microsoft isn't and hasn't charged unreasonable sums for their products, and they have demonstrated that they understand in order to hold onto their 80% (less if you count the installed base of Linux being used as a 'consumer desktop OS') or so share of the consumer desktop OS market, their software must continue to be a marginal cost in the total cost of the PC.
Intel and Cisco are two other companies that have come under focus for their dominant positions; neither charge outrageous prices for their products, and if both prefer to hold onto their market share, neither will charge such prices anytime in the future. Neither MS, Intel, or Cisco are coercive monopolies; and it's just more proof that a free market does not create and maintain harmful monopolies, only the government can and does. That Linux is gobbling up share of the OS market (mostly the server market) very rapidly is more proof that Microsoft has no method to truly block competition (as Microsoft would like nothing more than to block Linux); Linux however, does not compete well in the consumer desktop OS market, mass consumers do not want to use Linux yet. When or if Linux is able to compete well in that area of the OS market, Microsoft's dominant position will be threatened.
In regards to your theoretical - if you're behind your payments to them, and you've previously agreed to a license that says they have the right to shut down the software if you don't pay them for their product, then it is your responsibility to understand the agreement you are agreeing to when you use their product/s. If you click the 'agree' button without reading what you agree to, then you hold complete responsibility for complying with whatever is in that agreement. I'd like to think that most people wouldn't sign a *paper* contract without reading it first, much less not read it because the document happened to be long and they'd prefer not to.