Hello!
Did you use any other antivirus or antispyware in your computer? Did you use in the past?
Which is your Standard Shield security level? High or Normal?
avast should protect you against virus before they change register or hosts file and avast has a repair function to executables that could be repaired. Did you finish VRDB (virus recovery database) generation?
a generic behavior blocker is not implemented in 4.x version, but it’s planed for version 5
and what about VRDB? it can handle simple repair operations on system related files… but it’s always important to keep your own backups (no disinfection is perfect and you can observe problems other than virus related - some HW errors etc)… simply, avast is not a product like ghost or true image