What other security based software do you have that might run on boot, e.g. Spybot S&D (TeaTimer), AdAware (AdWatch), SpySweeper, PrevX, WinPatrol, ProcessGuard, etc. ?
Have (or did) you have another AV installed in this system, if so what and how did you get rid of it ?
What is the sensitivity of the Standard Shield (default Normal), the higher you have it more files scanned on boot ?
The more applications you have running from boot the more files will be scanned. So ensure only essential apps are running on boot.