^ problem in that method is that even the virus or rootkit will get copied to whatever device he's using to backup his files. then he'll get the virus again when he attaches the usb hdd to his computer to copy back the files.
best option would be to remove the offending virus or rootkit first from all devices like usb drives, external hdd's, and hdd's of both his and his brothers computer. then take a backup of all required files, and then last after checking everything is there, format his computer if he wants to.
another way would be to stop using admin account and make a standard user account in win7. people usually log in as admin and disable uac. uac is the first line of defence against such infections. disabling uac is like keeping the door of your home always open. use the standard account to browse internet or play games or download what every you want. keep the admin account clean.
best option would be to remove the offending virus or rootkit first from all devices like usb drives, external hdd's, and hdd's of both his and his brothers computer. then take a backup of all required files, and then last after checking everything is there, format his computer if he wants to.
another way would be to stop using admin account and make a standard user account in win7. people usually log in as admin and disable uac. uac is the first line of defence against such infections. disabling uac is like keeping the door of your home always open. use the standard account to browse internet or play games or download what every you want. keep the admin account clean.