-There's a lot of software with no equivalent in other OSs (including computer games, which are a selling point for a lot of people, and Visual Studio)
-Many companies are reluctant to change software they've been using for any length of time. Changing the OS is specially unlikely.
As someone who jumped from Windows XP to Linux (Knoppix, Ubuntu, and Arch) and now back to Windows 7: 1) No driver issues. I've heard bad things about Nvidia Optimus drivers for Linux. 2) I don't have to worry about not being able to install software, or other compatibility issues. 3) Games (the Mac gaming situation is pathetic). 4) It doesn't force me to muck around with the system all the time. 5) Windows 7 is actually really pleasant to use - much more so than XP or OS X; out of the box, it's only a bit less pleasant than Arch after I've spent a day installing and configuring software.
My misadventures with linux include forgetting to set up a netbook to connect to unencrypted wireless networks and the latest kernel update breaking wifi after suspending or hibernating; I've never had to deal with any of that on Windows. The most time that I've spent on system administration has been switching to an SSD, which became extremely easy after I found the right software to mirror my disk.
Because it comes on most PCs, runs most applications, and on the whole works "good enough (TM)"
The fact that the malware has to bury itself so deep into the boot sector actually says some pretty positive things about Windows security. Nothing like those heroics would be needed to target Linux for example (I don't know about Mac).
1. Ubiquity
2. Honestly - better user experience in Windows 7 than any Linux desktop I've used, but then I go back 20 years with Windows
3. Any document-handling industry is so deeply into Word that you really won't be switching any time soon. Sad, but true.
-There's a lot of software with no equivalent in other OSs (including computer games, which are a selling point for a lot of people, and Visual Studio)
-Many companies are reluctant to change software they've been using for any length of time. Changing the OS is specially unlikely.
-It's really not that bad an OS...