How to give bootscan in my PC?

Hi! Friends,
I have been having a great problem with virus attacks on my PC.
I have Avira Antivirus on my PC and when I scanned the whole PC, it says that some files are locked and can’t be deleted.
I think they are system files.
So, what can I do, if I want to delete these viruses from the PC?
Do I need to set up my PC or is there any other solution for that?
Please help!
Thanks.
