Microsoft Windows is the security hole in your network and desktop.
After experiencing security vulnerabilities in Windows such as the notorious ILOVEYOU virus at work, I started researching and looking into network security seriously. Between January 2004 and June 2004 (5 months!) there were about 4000 viruses that attacked Windows. If you purchase a spyware removal program, you will find hundreds of malware on your Windows system. Some of these malware are actually very serious that they transmit your key stokes on the keyboard revealing your passwords and any banking transactions.
Microsoft is costing their customers lots of expense to patch their systems and try to protect themselves from Windows' security vulnerability. This is also reported by many Technology experts such as Gartner's report on Microsoft's Security holes.
The following is a link to their report: