Thank you all for your answers.
It was a bit silly of me to ask without telling you what kind of computer I have
So I have added some info in my signature. Wasn't really sure what to write, so I hope I haven't missed anything important.
Yes it did find a problem when scanning, and I think it's cleared up.
I defragmentated my PC yesterday. It took forever too. I put it on at midday and it still hadn't finished by 1am. But it was in the morning
So maybe Avast will be able to scan faster now