on-Demand ("Normal" scan) you should enable archive scanning, but the ResidentShield (On-Access-Scanner/Background-Monitor) shouldn't scan archives as
- malware in archives is inactive until unpacked, and then the Shield will catch the extracted file
- imho it would pretty much slow down your rather weak machine
- setting the avast shield sensitivity to "high" would be a better approach to enhanced security..
- and read the link "VirusRemoval" below in my sig about how to secure your system better..
you might want to browse the forums here a bit, especially the User's FAQs in OFFTOPIC forum, and read the avast help/docu/FAQ's