How to convert my music to ringtones for the Iphone 5
GarageBand can do it. Import a song, select a 30-second range, export it as a ringtone. 

http://support.apple.com/kb/HT1358

Edit:just saw the PC bit.

Haven't tried it, but there's an app: 
https://itunes.apple.com/au/app/ringtone-maker-make-free-ringtones/id390929278?mt=8

And a website: www.iringer.net/