I have also experience the same problem on 3 laptops with Windows 10. Sometimes (not all the time), taskbar starts to freeze, Start Menu not working, when Avast Free is installed.
I did some googling and found some forums talking about Avast as the culprit. I did uninstall Avast on 1 laptop and problem solved. No freezing of taskbar, start menu is working fine. I uninstalled Avast on the other 2 laptops and replace it with some other free AV and problem solved.
I still use Avast Free on my desktop computers with Windows 8.1. I just don't use it on Windows 10.
Last time, I ended up reinstalling Windows 10 as I thought I made a mess. Installed Avast, then problem is back again after some hours of usage. That's where I realize that it could be Avast's fault and did some googling and found out others having the same problem with Windows 10 + Avast setup.