Hi hilroy89,
Think of an archive as a box of office or household junk neatly packed away for transport or storage: books, files, tapes, CD's, pictures, old magazines whatever.
To get at any individual item in the box, you need to rip open the seal and unpack every item to get at the one you want.
It's the same with a computer archive: avast! can tell you if there's a virus in the box, but you have to unpack the box yourself if you want to delete it.
An archive is often a compressed file like a .zip folder. If you want to get rid of the virus, you'll have to extract all the files in the folder and delete infected ones individually, or delete the whole folder.
Viruses in archives are not active, but you need to think about where the archive came from: if it's something of your own (zipped documents etc.), you will probably want to extract the files and delete or repair any infected files.
If it's something you've downloaded from the web or which arrived as an email attachment, you would be well advised to delete the whole archive unless you are very sure of the source.
What exactly was the archive in which the malware was detected and where did it come from?