Extreme CPU Usage on boot

For the past couple of days, whenever I boot my system, it is extremely slow for around 5 minutes, and when I open Task Manager, it shows I am at 100% usage, with Avast Service using up every single percentage it can get its hands on, which likely explains the slowness of my PC. When I try to open Avast, nothing happens until the high CPU usage stops. After that, it fluctuates between 10-50% usage.

I’m currently working on a full re-install as repairing it isn’t working, but has anyone else experienced this? I’d be lying if I said I wasn’t highly concerned.

EDIT: Hard reinstall appears to have fixed this, but will update if it returns.

Which operating system and version?
Which version of Avast, Premium or Free, and version?