PC and PIC must comunicate together at least each few mSec to keep communication alive, that's USBSERVICE's job, but you can send your data when you want, the way you want. See USBDemo in code example section, and the famous POST #148 where Darrel gave a way better solution to handle USBSERVICE, save loads of headaches, run in background etc etc etc
http://www.picbasic.co.uk/forum/show...&postcount=148

HTH