That is where avast unpacks archive files for scanning and the unpxxxxxxxx.tmp is an unpacked file being scanned. On successful scanning the unpacked files should be cleared from the _avast4_ folder, so there was a problem because it wasn't deleted after the scan.
You can safely remove it from the chest, it isn't a virus, but adware, usually responsible for delivering adverts or browser hijack, etc. But, it normally wouldn't be in that location and avast should have alerted on the file it was extracted from.
