Originally Posted by prathmann

Replace the chain. If you find that the new chain slips on the old cassette then replace the cassette as well. It would be quite unusual for a chainring to be worn in only a couple thousand miles - note that the rings frequently have some of the teeth made with a different shape to facilitate better shifting.

Although the rate of wear is very dependent on your riding style and conditions here's what I find to be typical lifetimes for the different components; chains: 4000 miles, cassettes: 15000 miles, chainring: 100,000 miles.