Virus definitions for Avast version 5-11 are indeed not created anymore (they are not compiled, not signed, not released - they simply do not exist).
The techspot.com page only makes it appear they are, most likely because they are showing the release date of the 18+ definitions (i.e. of a program version that is current and supported). But the definitions you actually download from there for Avast 5-11 are old, they won't update anything.
