Not having the Dutch version, I can't say you haven't got it. However, I believe this is an integral part of avast! regardless of language.
The only thing that would stop its use is if you don't have an OS that supports it, e.g. win98, winME. It is only available in winNT win2K and XP, the option is greyed out if your OS doesn't support it.
What OS are you using?
Where were you looking for 'Schedule a Boot-Time Scan' and was it greyed out (faint text color)?