Let me get this straight......you have the ipod with all your music but not the old pc? If so, download itunes to the new pc and use something like copypod, which I used to get the music off my ipod when the Hard Drive failed. If you have the old PC and want to use the new PC as well, download Itunes, then set the Ipod to act as a hard drive, not automatically syncing. Your music from the old itunes wont be on the new PC, but you can use more than one PC for your ipod.
All you would do to copy music is connect the ipod, copy the music library from itunes, click to highlight your ipod on itunes, then click paste in the top bar.
Thats how i do it, and i can download music to my ipod from any pc i would have thought. The secret is to have the ipod set up as a hard drive, and not automatically syncing on connection.
Hope this helps.