I have had Linux on my PC for almost three years now, but some characteristics make Windows a better choice, thats why I have to dual boot Linux with Windows.
The main problem with Linux is that it does not support Windows programs very well, although there are substitutes, many the games available can not be played on it and what if you want to use the latest official MSN Messenger, with all its cool(but resource consuming) features? It wont be able to run it.
The second thing is that most of the Linux distributions are not as user friendly and straight forward like windows, the pros call it more features, beginners call it confusion. Running a Windows installation removes all the non-Microsoft Operating Systems, whereas Linux asks what exactly you want to do. Plus partitioning the hard drive during installation also confuses the beginners. Apart from the installation all the Windows stuff is considered standard, so Linux file hierarchy would look alien, so would the user interface and users would have to search where a particular option is located. Plus installing new programs is mostly not as easy as downloading a single file from the Internet and pressing the ‘Next’ button again and again till the installation is complete.