Hi, xenos, and welcome to the forum.
There are quite a few likely causes for a slow startup. The chief suspect is likely to be malware, given what you've said, but there could be a couple of other things worth checking, first.
Did you have any other AV installed prior to Avast? It would need to be completely uninstalled, using a specific removal tool if necessary, and then maybe Avast repaired.
What was the name of the malware detected? Either the infected files (names) that were removed, and/or the name that the rogue called itself.
Can you please post the most recent MBAM report. It can be copied and pasted in your next reply.
What is your OS (including service pack), and the computer CPU and RAM?
I suggest you clean your temporary files (using the built in disk cleanup utility), and run a boot time scan with Avast, to help remove any leftovers.