... After I reinstall avast it works perfectly, but when I restart it stops working.
Please excuse this stab-in-the-dark from this newbie, but since your problem seems to have similar symptoms to mine, and my machine was also a Win7 Ultimate, you could try scanning your hard drive for bad sectors and bad file indexing before doing a clean uninstall and reinstall. (Yes, that was how I fixed my system.)
Go to your Start menu > All Programs > Accessories folder, right-click "Command Prompt" and select "Run as Administrator". Assuming you have a standard Windows installation where everything is in your 'C' drive, in the Command Prompt window, type without quotes, "chkdsk c: /f", and press Enter. If it asks to schedule the check for the next time your system starts-up, tell it
Yes. (If it ever asks to unmount or dismount the volume/drive, tell it
No.) Then reboot to start the scan. If you intend to install Avast in a different hard drive, might as well do a chkdsk /f on that drive as well (do it after the reboot, and change "c:" to your other drive letter). Since the "/f" tells chkdsk to fix all errors, feel free to run aswClear.exe and do a clean install of Avast8 afterwards.
BTW, I was given this version of aswClear.exe:
http://files.avast.com/test5beta/aswclear.exe. I was told this does version 8.0 and earlier, and the nice thing about this aswClear is that you can run it in Windows' normal mode, in which case it would ask if it can reboot Windows into Safe mode. Of course you should choose yes, and it will automatically run once your Windows had rebooted, and restart Windows after it is done.