The traditional way to bet against a company's stock is to [link||short] it. That is, you buy a contract to sell shares at a particular price on a particular day in the future. If the stock price is lower on the day the contract ends than when you started, you profit. If the price is higher, you can lose your shirt. It gets complicated if dividends are involved, if it goes up too much and your brokerage account doesn't have the funds to cover it, etc. See the link above.

Short selling is very risky. And to win you need to be correct in your bet on the direction the stock moves and correct in the timeframe.

It's especially risky to bet against MS. They're still making a huge profit every month even if they have more competition (Linux, OO.o) than they have had in many years. If Windows and Office tank, they still have enough money to buy up some other company and remake themselves. You're essentially betting against the casino when they can change the game at any time in shorting them.

If you like risk and want to bet against MS, you might want to look at Novell. If you really like risk, look at [link||Red Flag Linux] - China's going to be a big Linux market and a home-grown variant probably has a leg-up. Diversify though. Don't put all your mad-money in one place, and, as always, be prepared to lose it.