I’d love to help you more, since I had the same sort of issue, but your problem sounds very specific to your environment (otherwise there would be a lot more people screaming for help on this same problem!)
Are you having trouble on all Vista machines or just one? Is it a factory build Vista image or a clean Vista install? Please provide more info on the environment if you can. But I’d suggest a chat or email to Tech Support for further help on this one. Maybe they can guide you through some local logs.
If you haven’t tried already, pick a machine to test with and uninstall then use avastclear to clean up completely (http://www.avast.com/uninstall-utility). Do all the usual things to check there’s no other security products installed that might conflict. Download the FULL installer from the cloud again to be sure your installer is not corrupt too before trying again.
That’s how it look like ( it’s not a safe connection problem only ):
Okay, so we’re dealing with one machine so yes it does indicate a conflict of some kind and eliminate the installer as being the problem for now.
Can you tell us the make and model of the Vista machine?
The experience I had can be seen in the thread https://forum.avast.com/index.php?topic=171539.0, where it shows that updating a driver from around 2012 to it’s most current (which I think was 2014 from memory) fixed my issue. So without any other suggestions of a solution I’d head in that direction again 
You’ve already indicated that you have the most current Realtek driver; is it an onboard adapter or 3rd-party, and which chipset? There’s a possibility that the generic driver is not working as happily with the card as you might expect. Realtek don’t make adapters, and vendors sometimes tweak their hardware to work better with their own drivers, so we should look there first before the generic. You could try downgrading this driver to the built-in Windows Realtek generic driver and see what happens too.
Similar idea for the Linksys AE2500. Although the latest driver, it is still from 2011. Windows probably doesn’t have a built-in driver for this, but I see on http://www.linksys.com/us/support-article?articleNum=148503 the Windows 7 and XP driver is the same but Vista is different. ??? You could try installing the Windows 7 driver, it should work.
In both cases if neither fixes the problem you can always switch back to the original driver.
First of all: Thanks a lot, GFM, for all your help!!! ;D
Yesterday was a busy day…
1º - made an Windows Vista backup image;
2º - install Windows 7 in the same machine, update it: tried both 10.2.2505 and 10.3.2508… NO problem!!! Web was mine!!! 
3º - use Vista image to restore;
4º - 10.3.2508… lost web again… :-X
5º - 10.2.2505… surprise: Webshield ON and web was mine again!!!
Conclusion: 10.3.2508 + Vista = 
10.2.2505 + Vista = ;D
Who’s the bad guy?!.. :
You’re welcome, I will help where I can, I’m only sorry that forums isn’t the speediest way to do things :-[
Would you be willing to try a fresh Vista install for testing, with no 3rd-party driver installs? Really all you’ve shown here is that your Vista backup image doesn’t work with the newest Avast. Also the make/model of machine could be of use.
Inconclusive; you haven’t changed my mind yet that old drivers or old 3rd party applications aren’t to blame Yes, its easy to blame Avast versions, but it could be something else on your machine image that is badly behaved and conflicting, and fixing that resolves the root cause. My experience as I documented in my other forum post is evidence of that.
May we ask, are you using Vista for a particular reason? Your other machines are Windows 7 or better. Seems like an odd one out, and Vista is… well… notoriously “difficult”.
I’ve always been satisfied with Windows Vista since the first time I had contact with this operating system. I never had the problems that many shouted over the years. For the rest, if I paid for it, it is expected that I make use of the product, is not it ?!
Fine, I’ll do a fresh install ( just an up to date Vista, nothing else )… but I must say: results won’t change. Why?! Simple:
Conclusion: 10.3.2508 + Vista = :'( [b]10.2.2505[/b] + Vista = ;D
I’ll be back as soon as possible!!!
And… As I said earlier: results did not change…
Something between 10.2.2505 and 10.3.2508 has changed and “create” the problem…