Outpost Pro, ant-spyware plug-in will really impact on boot duration, it scans hundreds of files and because of that avast will scan the files it want to open for scanning. The Outpost anti-spyware plug-in is based on the AdAware program and if you have other anti-spyware programs installed then you are well catered for without the OP plug-in.
The sensitivity setting of the Standard Shield can also impact on boot duration, Normal (the default) provides a good balance between protection and performance.
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 have an impact on boot, e.g. SpySweeper, Spyware Doctor, PrevX, WinPatrol, etc. ?