Thank you very much for your answer.

TwinHeadedEagle,
with time, the problem is solved. It went on 3 hours scan time … my computer is runing well again.
I noted your solution ; it can be usefull another day . I Will download this tool.
My hard drive is a velociraptor 250MB, i imagine the problem with a 1To HD as system drive.
I have seen that avast was scanning $recycle directory , it wrote 0% for a long time : why a directory containing deleted files ?

DavidR
avast as been installed for 3 years, my problem happened after a detection of webcake that is consided as a malware.

avast should give an estimation of time scan when it propose a reboot with scan

F.P.