What Operating System are you using ? is it up to date ?
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. ?