Hacker, virus – how can I remove malicious codes?

Dear all,

Recently my blog http://www.saxemetais.com.br was hacked and google sent me an email alerting about it.

ok. I put an alert screen that we were under maintenance and now I have to get rid of it.

Questions:

If I change the Theme (psw etc) it would solve the virus problem?
What is the best way to let the WP safe of hackers?
What is the backup system recommended by the forum specialists?

Thank you for your help

Daniel