This 'old' topic is also related to having previously had Kaspersky installed so unless this exactly mirrors your problem I would suggest you start a 'new topic' of your own and answer these questions.
What is your OS ?
What version of avast do you have, the latest is 4.8.1201 ?
Have (or did) you another AV installed in this system, if so what was it and how did you get rid of it ?
What other security based software do you have that might block new startup entries, e.g. Spybot S&D (TeaTimer), AdAware (AdWatch), SpySweeper, Spyware Doctor (StartUpGuard or OnGuard), PrevX, WinPatrol, ProcessGuard, etc. ?