Your operating system will keep certain files open for exclusive access at all times. No application within this operating system can access these files, ordinary applications like virus scanners and viruses alike. The files vary slightly depending on your operating system. This is the most likely reason why your scan report tells that some files could not be scanned. Neither will the virus scanner get access to encrypted or password protected files. This is perfectly normal, and you should not worry about it unless your machine clearly shows signs that it is infected (e.g. automatically sends e-mail).