Copy it from the old PC to the iMac.
There are a variety of techniques including Migration Assistant for Windows, enabling file sharing between the two computers, or copying your music to a flash drive or other storage device followed by importing it to iTunes.
On the Windows machine, download Windows Migration Assistant here: Windows Migration Assistant v1.0.1
Instructions are here: http://support.apple.com/kb/HT4796
Once your music library and everything else you want to transfer from the old PC is on the iMac, sync your iPad to the iMac:
You will choose Erase and Sync which will do exactly that, in that order.