If you read the fine print of their software license agreement, you'll see that the code comes with no guarantee to work at all and the developer is not responsible if it crashes your system. Operating system designers do need to keep their systems secure so they continue to sell, even if they aren't under any legal obligation to do so.
Every operating system has security issues.
The only sure way to keep intruders out is to turn the systems off. That said, Microsoft has a long way to go to keep not only its operating systems up to par with the industry but also its applications. The recent outbreak of the Melissa virus, which exploits the Word and Excel macro language by quickly replicating a pornographic e-mail message, goes to show that someone at Microsoft was asleep during this feature design. Melissa is not the only macro virus in existence, but one of the first of many pernicious attacks that will continue to frighten end users and drive corporate computing departments crazy. These macros can do very powerful things, like starting applications and writing or changing any file on your hard disk.