of course backup is the magic word
the troubles i had was first to correctly transfer the bitcoin-qt files [ may be i used to much command line options...but i will retry from zero ]
second trouble is to have bitcoin-qt uses a defined wallet , all it does for me is to create a new wallet .
may be the wallet = bank account could be , like paypal , connected to a email account , that would be simple for web shop to use bitcoins and easier to remind than all that letters ...
google already having some very good tricks to prevent the email account to be hijacked