Hi all,
Can someone help me ?
After the scan i get a message telling me that it cannot scan files protected by password. How can i scan those files ?
Thanks 
You can’t, unless you extract them first somewhere. avast! cannot guess or brute-force the passwords.
Viruses (if any) inside of password protected files won’t harm your system if you have ‘on-access’ scanning turned on.
Anyway, password protected files are generally viruses signatures of other antivirus program, antispyware signatures, etc. Their own programs encript the signatures for safe and avoid ‘false positive’ detection of other programs (antivirus).
If I send you an email with a password protected attachment but neglect to give you the password, you can’t open it (or rather it would take a very long tome trying to crack the password). It is no different for avast!, it is possible (with the right software) but you wouldn’t want to wait for that long during a routine scan ;D