It's not popular for the same reason why Microsoft and Apple market their stuff so heavily. The typical PC user isn't well informed of the free operating systems. Truthfully, I feel as though it's due to an ease of use issue. Everyone knows just about everyone supports Windows, and Macs have been well known and loved for a while so that's why. Linux is starting to gain it's market as well as more and more of the less technically savvy people find out about it. I've shown a few of those non-technically savvy people Linux before when they manage to FUBAR their Windows install, and they were surprised on how quick it was running off of the CD drive on their PC, and how easy it was to get their programs. I told them that for the most part, you can run most Windows applications in Linux (you can) and that some programs have to be installed through a command line, however there's loads of help over the Internet plus I support Linux so yeah.
But you won't know what kind of problems you'll hit until you install the Operating System. If you have a fast Internet connection, you can download the CD/DVD, burn the ISO to a disk and then boot up the disk without installing anything to your hard disk. You can give Linux a shot there and see if you like it. The only real problem you'll have with switching to Windows is of course, getting your older Windows applications (if none can be found for Linux that are similar) to run under programs like WINE, and of course drivers and backing up your files. But as I said, give it a shot. If you want to have more of a hands on approach to learning Linux while keeping Windows intact, dual-boot Linux and Windows or install it to a Virtual Machine
EDIT: Also, a lot of people still think that Linux is a command line based system, much like what you'll find on older Vintage PCs running Basic where you'd have to tell the PC to run a program in a certain location, or enter in instructions into the computer yourself. That's not true. Linux (as well as Windows and OS-X) can be GUI based or Command line based.