Please keep in mind that this list is very broad and covers flaws in more than one version of windows operating systems.
1) Microsoft uses monopolistic approaches to business. They started with OS detecting in code Windows 3.1 so that Windows would only run on MS-DOS and not any of the DOS type operating systems from other companies. This behavior has continued in different forms ever since? Anybody remember Netscape?
2) Windows comes pre-installed on most computer systems intended for home use.
Windows is full of security flaws that in combination with it's domination of the market make it a huge target for hackers and malware. It's like painting a big huge target on yourself and running through a shooting range.
3) Windows is one of the few systems that actually has normal users logged in with root privileges all the time. If you aren't, many applications will not run properly if they run at all. There is no real sudo functionality in windows unless you install and configure third party software to do the job. Sure, Vista at least nags you and has slightly better support for limited accounts, but people will train themselves to just click accept without heeding the warnings. Being logged in all the time with root privileges means that malware can run and delete or modify important system files without you even knowing. The life of malware on Linux, BSD or MacOs would be very difficult because it wouldn't have this access.
4) Many many many security flaws - too many to list them all. There was an RPC flaw that allowed access computers connected to the internet during the install process. IE's ActiveX is full of security issues. These are just a few of the well known security flaws in various windows systems. If you don't shell out $$$ for the latest and greatest subscription to a full anti-malware solution (not just virus scanning) you could be a sitting duck with windows installed.
5) It's not open source. Some of us want to know what kind of code is in the software we use. Some of us want the ability to patch problems on our own if the need arises. Besides, if it's not open who knows what kind of nasty things like back doors could be in the software? Open Source applications allow people to identify and correct problems with the software and to submit their own enhancements and features, as a result updates are more frequent.
6) Windows Genuine Advantage. I realize that Windows is probably the most pirated software in the world but at the prices charged with the near monopoly status of Microsoft is anybody surprised? Maybe if Microsoft lowered the price of windows more people would start buying Windows instead of using pirated copies. WGA is not the right approach to fight piracy, anyone with access to google can find ways around WGA or activation for that matter.
7) Linux can be used with minimal hardware requirements. You can run Linux on almost anything. Switching to Linux might allow you to keep the hardware you have now for another few years and keep some electronics out of landfills. The increasing hardware requirements of Windows upgrades turns the PC into a highly disposable platform. No wonder so many people are buying macs.
8) Linux is FREE. Windows is expensive. Linux systems can be used on limitless machines without paying additional fees for each installation or computer.
9) No single person or organization controls Linux.
10) Why should anyone pay to cripple their computer and themselves with DRM when you can just use Linux? DRM is supposed to be a way for copyright holders to prevent unauthorized duplication of their work but what it actually does is restrict how you use digital media like music, or movies in ways that are not included in law or licensing of the works.