The notes are public domain, but the work of an orchestra to transform them into music is protected by copyright. Violinists gotta pay the rent too ya'know.
Try Napster: for $10 month you can legally keep thousands of CDs on your hard drive in (actually quite decent) 128 kbps format. Try the 7-day free trial, though it requires a CC number for the "premium" version that lets you do this (lite = like itunes store, you can only buy music).