It's not just the upgrade, I did a fresh, vanilla install of 8.1, installed avast enterprise only, rebooted and got a bluescreen after the install. On two computers.
Only way to get it to boot up was to go into safe mode and type this:
sc config aswsnx start= demand
Frankly this is something that most home users can't/won't do.
Whether or not they admit it, Microsoft is probably removing avast on purpose because they know that if they leave it installed, they'll get even more support calls and complaints about a hung update, or worse, a bluescreen after the update. (For the record, on RTM day, I couldn't even upgrade my home pc from 8 to 8.1 until I removed Avast entirely, otherwise the process would hang at a "Getting things ready..." screen and the update would rollback - I tried three times, then removed Avast and it just worked. Properly.)
I'm sure that they're doing it this way to ensure that their update completes successfully, and to highlight the fact that everything works fine until you re-install avast. Then it goes downhill and you have a clear picture of why.
Sure, finger-point and blame the Microsoft upgrade process, but at some point you guys (Avast) need to realize that Avast Business doesn't work on 8.1 at all and given how open the access was to the preview version and the RTM, you have to take some responsibility. Furthermore, the fact that the free version works when the business versions don't is a joke.
This is like the Windows Update Please Wait loop all over again - yes, I get that you think that MS should change how they're implementing pieces of their software, but news flash, they won't. Why would they negotiate or work with you when there's a ton of other antivirus vendors who manage to get this stuff right without handholding from Microsoft. Even if you think they should do it better, there's obviously ways to make it work without their intervention.
On that note, since Microsoft's strategy seems to indicate that yearly updates are going to replace service packs, I can't afford to wait for avast to negotiate with Microsoft everytime an RTM blows up your software. It's unrealistic and will delay implementations. As a avast partner, I'm not going to start telling my customers that they can't upgrade their OS because avast doesn't support it yet - the response to that will be: "Well, I want the latest and greatest os. What about ____(insert competitor's product here)____? Will their product work?"
You need to understand I like avast, I'm proud to support it and promote it, but when you do stuff like this, it's embarrassing. Standing by and recommending your product when you make such obvious misfires will eventually make my customers come to second-guess my suggestions. I don't want to sell your product, I want to deliver my customers the best possible solution. For many years, it was easy to do both. I could sell avast and sleep at night because it was the best possible solution. This was win-win. Now, I'm not sure and neither are my customers.
If I didn't like avast, I wouldn't even bother posting - I'd already be looking at other products, but you need to understand that this criticism is valid and that I validly want to see you succeed. To that end, you need to lock this down and figure it out before 8.2 or 9 or whatever comes out or I suspect that I (and many others) will start looking for products that aren't going to delay my software implementations or bluescreen a fresh install.