Maybe avast is not being able to delete it…
Can you schedule a boot time scanning? (right click the skin and choose the option)
It’s good disable the System Restore too:
Start > Control Panel > System > System restore > Disable
Click Apply
Enable it again
Click Ok
Schedule all files (including archives) in all hard disks.
I suggest you choose the ‘manual’ operation (‘Ask for action’) until you get familiarized with avast.