Well, I think I too read too much horror stories here and was pretty much as worried as you. I read all the methods and the potential fixes. I wondered how in the 21st century, one cannot come up with a clean installer that does all the work better than a human. After much debate inside my head, and remembering that the first time I boarded a plane was with the clear intention of jumping overboard and not land with it, I told myself the h*** with it, and again, lets jump; and I clicked, almost with my eyes closed, on the “install” button at the bottom of the free version column on that harassing window, after restarting my computer (to get that window to show up again and be pretty sure nothing else than my internet connection was running).
I didn’t remove 4.8. I didn’t even stop it (hey, after all, I’m still connected to that Petri dish we call internet). I saw the download happening then I clicked the Install button (probably after saying yes to some sort of licence, but I don’t even remember as I may have been has nervous as some machine gun meat landing on the beach on D Day!). I saw the installation begin, quickly, the box told it was creating a restore point. The nervosity went down by a notch then. I saw the 4.8 icon getting slashed out. And it installed. I rebooted as asked. The moment of Truth was near and my computer came back to life. The nice new icon was there, but slashed out. OK, I went into the settings and made some changes to the default to get them somewhat higher and to activate the thing. And yes, those predefined scans were inactive too as some others reported. I rebooted again, the icon came back slashed out again but the predefine scans were now fine. I reactivated the program and rebooted one last time, and now, the AV comes back active each time. IT WORKED!!!
I even think the sucker is faster than 4.8 and the update message is somewhat less “aggressive” than the previous one. Checked my CPU usage and it actually went down by much. So, to counterbalance some of the horror stories we find here (I know, it’s an help forum and when one doesn’t need help…!) I thought it would be nice to tell you mine, though it has not much in terms of cliffhangers and ends so well it is actually quite dull and it won’t make it into Hollywood for sure.
Don’t know why my profile setttings don’t show as much as in the help section, t’may be cause this is my first post here, but can’t find how to make a sig for now. So here’s the machine in plain text:
Dell Optiplex GX240, 32 bit Pentium IV 2 GHz, 1 Gb RAM, Win XP pro SP3, Windows Defender, MBAM Free (so runs manually) and went from Avast! 4.8 to 5.0.545 without a hitch. I also scanned with the On-line scanner House Call from Trend Micro to make sure the other programs I have didn’t miss anything.
Is the venerable age of this machine the reason why it went this well? I don’t know, but as I age myself, I find drastic changes are usually much harder to accept so easily… 
until I looked here). I have notifications switched on and all. ???