I may have missed it but, which edition (Free / Pro / AIS) are we talking about? Which version of Avast?
If Windows is not "up-to-date" (for example, Win 2k with SP4 at least) then update it before reinstalling Avast.
How much RAM and free HDD space do you have? Have you check that you get to at least the minimum resources needed by the current Avast?
The current stable version of Avast is 6.0.1203. You may download older free stable versions from
http://www.filehippo.com. (BTW, the old version 4.8 is still supported too, for now.)
Please clean you web browser's cache.
If it is not "too difficult", and you are sure that other components (soft and hard) are working correctly (no corruption of general files / file system), then I would suggest a new download, probably of the previous stable version (6.0.889 I think) instead of the latest, and then use the Clean Uninstall Utility from
http://www.avast.com/uninstall-utility directly under
Windows Safe Mode.
Then reboot in Normal mode and install anew using the new downloaded setup (again, better to start with the previous stable version instead of the latest one).
In Avast -> settings -> updates -> select the option to use "direct connection (no proxy)". This may solve some problems when Avast updates the database.
Then try using that version for some days, updating the database and engine only (not the program). If in this situation all works correctly, then keep using that version with normal database updates at least until the next stable is ready (and then wait for some additional days too).
If any of these questions and/or steps are not clear enough or you get some problem in the process, please come back to ask, reporting the details of the problem.