Oh darn XMAS, you gave the answer away.
Though it's listed on the "News" page, which is accessed from the home page, as an alternative, I've been patiently waiting to see if anyone would actually search "avast vista", or something similar on Google. That's the first page that comes up in the results. You can then backtrack from there to see where it's at on the website.
It's also the first item on the 9/25/06 revision history page for version 4.7.892.
Sounds to me like avast has it covered.
Edit:
Oh, and dovetailing off of xistenz's post, wouldn't offering a "free trial" to the free version be a bit of an oxymoron?
I believe I understand why avast isn't listed there, but hey, maybe I'm missing something from the "concerns" presented in this thread...