Hi Roy,
Googling the error number and looking at some of the responses suggests the download is corrupt. It's also suggestive of a type of firewall that seems to be associated with this; the nVidia Forceware access manager. (I've downloaded this myself, unintentionally, while updating nvidia drivers; it installs by default.)
I would suggest that if the above is true for you, uninstall the nvidia firewall, (other firewalls work OK here, including Comodo, Online Armour, Kerio etc, or XP's own firewall which only protects inbound though,) re-download the Avast installer file and try again.
The XP firewall is fine as a stopgap measure while downloading software from known safe sites. Some people use it all the time. I think most of us are happier with two-way protection, though.