If your music is in mp3 format you should be able to drag and drop through windows explorer from the iTunes music folder (where you keep you keep your music rather than the iTunes application). If you want to use an actual program, Windows Media Player should recognize it. I actually use Rhapsody, the player and the to-go subscription service It's $14.99/month if you pay month-to-month; I pay $39.99 per 3 months.
As far as music purchased on iTunes, the Rhapsody program will only play iTunes Plus (unprotected), but I believe the Clip doesn't support even them.
I searched around a bit and couldn't really find a good answer. I've got an iPod but I am by no means an expert when it comes to MP3's. When I got the Clip I assumed that I could just use it with iTunes but apparently that is not the case.
So, what do you use and can you put music from itunes on there or do you have to use a different music service?