I'm assuming that you installed iTunes on your PC and want to use the same library for iTunes you use for WMP.
Download the latest version of iTunes from www.apple.com/itunes, and install it on your PC.
When installing iTunes make sure that you uncheck the box next to where it says import media into iTunes. Also uncheck the box next to Keep iTunes folder organized.
Locate your music folder on your hard drive: browse to your WMP music folder by right-clicking on any music file and choosing > open file location.
Open iTunes and go to > File > Add folder to iTunes. Go to the same folder directory as where WMP saves music.
Open WMP > Organize > Manage libraries > Music
Click Add to browse the folder location of you iTunes library. Add the folder and click ok.
Now all of your music in WMP will show up in iTunes, and anything you buy from iTunes will show up in WMP. Just be sure to uncheck those two little boxes in iTunes during install.