We really need more information to be able to help fully.
- What detected these?
There is a deceptive trick of placing files in the system folders to trick and stop people from removing them, believing them to be important system files.
- What OS are you using? is it up to date?
- What avast! version and VPS file (virus database) number, e.g. 0436-4 (see about avast!)
- What was the virus name, what was the filename, where was it found
example (C:\windows\system32\infected-filename.xxx)?
avast takes a copy of these files to have as a backup in case of infection, are you possibly confused because these are in the chest (system files section), this is normal, see image.
- What actions have you taken to try and resolve the problem?