unlike the OP, i'm on xp sp2 [not sp3] and hence avast 10 [not 18] but having similar issues since rebooting after an update which may have been installed weeks ago since i use standby.
this seems wide spread, so i joined just to share my fix [took me days] which should work on
xp sp3/v18 too.
need [A or B]:
A] old avast offline installer
or
B] use system restore/recovery image backup to extract old avastUI.exe before this update bug occurred [why it even gets updated on just a database update with no program update is beyond me]
steps for A]
1.install old avast version
2.backup avastUI.exe from programs folder [incase it gets updated in step 5-6]
3.do a manual database update or download/install it from:
https://www.avast.com/en-au/download-update[untill this is fixed, steps 2-6 is how you will update from now on]
4.also turn auto updates OFF or it will freeze again after update/reboot!!!
5.reboot to safemode [to stop avastUI.exe from being updated]
6.reboot in normal mode: sweet - it works again! -if not go to steps 1-2 below.
steps for B]
1.extract old avastUI.exe from programs folder [compare version number to updated one]
2.reboot to safemode and overwrite newer avastUI.exe in programs folder with the old extracted one
3. do steps 4 & 6 above
4. for future updates, do steps 2-6 above
5. ....um, that's it i think? if any of this stuffs your pc it's not my fault!