I think my iMac disk may be a bit defragmented after three years of use including large video files. I did a test by installing a new copy of OS on external drive, and it was noticeably faster. This makes me want to try reinstalling Snow Leopard as a clean install. Can I then just use Time Machine to bring back everything *exactly* as it was? Will it do that? Emails and everything will be exactly as before? Many thanks.