Then insert a retail OS DVD or the restore media that came with your machine to reinstall the OS.
Edit: Sorry, I misunderstood. Apple Geniuses will replace a drive, sometimes in-store, if you have a problem with yours. Contact Apple Support and your local store for information. They will not (as far as I know) install a HD that you bought from a third party.
Yes for a nice fee! Depending on your model replacing the hard drive easy to somewhat easy.
For the white macbook's you just need to pop the battery and remove some philips screws, slide out the hard drive, and then install the hard drive bracket on the new drive (held on my torx screws). No unhooking of wires or anything.