So they are saying "Hé, if you want to use our product, put your system at risk"
They should change their software so it will work with all (major) av's i.m.h.o.
What exact OS/SP ?
What exact version of avast ?
Any other security (related) software installed ? (or was there)
What part of avast seems to be the problem ? (webshield, fileshield, deepscreen or...)
And IE 8 ?
Seems that you don't have your system up-to-date which can cause serious problems.
You should have IE 11.