I asked exactly what you are doing when this happens to try and pin down which provider is initiating the scan.
You don't say what your system spec is, I ask because, is it up to the job of all this multi tasking and AV scanning.
Sorry but if you don't answer the questions, I'm just guessing and that doesn't help either of us.
A reinstall has in the past overcome high CPU usage, if ashServ.exe sticks on 100% and the icon isn't rotating, indicating a problem, conflict/corruption, etc. This is possibly considering your comment "i have one problem with my avast in a last few days."
Assuming things were working fine before that, have you added any programs just before the problem occurred ?
Try a repair of avast. Add Remove programs, select 'avast! Anti-Virus,' click the Change/Remove button and scroll down to Repair, click next and follow. You need to be on-line to do this.
If that doesn't work try, uninstall, reboot, install, reboot.
It would probably be best to download the latest version of avast and save it to your HDD, somewhere you can find it again. Use that when you reinstall.