Your best bet would be to run disk utility and repair permissions/run disk first aid and see if that fixes it. If it doesn't, it's probably a hardware problem.

As far as downloading, torrents are the best way at the moment, Mac or otherwise. My Powerbook's hard drive is full to bursting with stuff downloading using azureus, a bittorrent client. You can get torrent links at various websites such as www.suprnova.org