How to convert my music to ringtones for the Iphone 5 [duplicate]
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/