I would suggest to let Avast scan dll's, not to override this. After changing that to the default (let Avast scan them), at least for troubleshooting purpose, maybe you could try:
1) Rightclick the Avast Icon in the Notification Area
2) Program Settings...
3) In the left panel, choose Troubleshooting
4) Check the "Delay loading of avast! services after other system services" option
5) Click "OK", close every other program and save your work and reboot.
This option may help you boot faster, but also it has the potential to let some malware get into your system and Avast not chatching it.
If this option helps you, you may try to locate what other services and/or programs are loading theirselfs. Avast is trying to scan those, and that slows the windows boot time. If you can identify the "problematic" proccess, another solution for you could be to delay *that* proccess, instead of avast. By doing so, the boot time will be fine, and avast will scan that other proccess only when it starts (that is, delayed).
I hope this helps.