The reason for asking about the sensitivity is if you had it on High it would be scanning more files. However, if the icon isn't rotating then it may have stalled as the rootkit scan should take seconds rather than minutes.
- Vlk, always says that the guidelines for solving of the problem "ashServ.exe using too much CPU" has always been the same:
1. Check if the avast tray icon is spinning (while CPU usage is high)
2. If it is, click it (wait for the dialog box to open), and check which file is being scanned (the Last Scanned field); the Scanned Count value should be increasing (spinning icon means a scan is in progress)
3. Find out which of the providers is scanning the file (click "Details" button and check out the Last Scanned value for each of the providers). The most common providers that may cause this are Standard Shield, P2P Shield and IM Shield.
Again this may not be to helpful as you say the avast icon isn't rotating, but it is worth checking.
Short of that revealing anything I would suggest a clean reinstall.
Download the latest version of avast
http://www.avast.com/eng/download-avast-home.html and save it to your HDD, somewhere you can find it again. Use that when you reinstall.
Download the avast! Uninstall Utility,
find it here and save it to your HDD.
Now uninstall (see below) using the Add Remove programs, reboot, run the avast Uninstall Utility, reboot, install the latest version, reboot.
It may be necessary to disable the avast! Self-Defence module (Program Settings, Troubleshooting section) if you are unable to uninstall using the Add Remove Programs before running the Uninstall Utility. Or, boot into safe-mode and start aswClear.exe (the uninstall utility) from there.