If they are in the C:\System Volume Information\_restore and a long string of numbers
You will need to do the following as the virus/s are in the restore points of the computer.
First go into control panel/performance and maintainence, towards the top of the window that comes up and on the left handside you will see "SYSTEM RESTORE" click on this, next click on "SYSTEM RESTORE SETTINGS" on the little window that comes up, click on "TURN OFF SYSTEM RESTORE". It will then warn you that you are going to turn off system restore and that all your restore points will be deleted (or words to that affect)
After you have done the above restart your pc and run Avast again and see if it finds anything, once Avast has finished, you can then turn back on system restore.
Regarding your other questions, you can if you wish delete the virus's that are contained in the chest, but whilst the virus's are in the chest they can do no harm.
I have used Avast for quite a while and I normally just manually run a full thorough scan every couple of days, im not sure if you know this but if you right click on a folder on your pc, you should see in the menu that appears a little blue circle with a letter a in the middle of it, this represents Avast, this allows you to scan that folder/file, for example, if you have downloaded a program. Hope this helps.