It took a little while to get through all your postings, and I just looked at David R's reply also--I see a genuine good effort there. However, let me suggest a slightly different approach before you delve into his suggestions. Either way, if you are keen on keeping the 5.0 effort alive, it is going to require the removal of 4.8 yet again in any case, and if it were to be another non-successful install of 5.0, a reinstall of 4.8 plus updates, which I see is a thorny subject for you. However, you indicate you already have the removal tool for both, plus the 4.8 and 5.0 programs already downloaded, so that will be your decision. Now, after looking at all the activity connected with this problem, it is my humble opinion that this is most likely just a boot problem, and not a major program issue, which seems to agree with David R's latest approach. I say this because you indicate that 5.0 downloads just fine, plus it will update and so forth as it should, AS LONG AS YOU DON'T REBOOT. That shows me that your system is fully capable of accepting AND running the program. When you reboot, you say the icon shows up, but after a short delay, it goes into the vicious reboot cycle. For some reason, 5.0 will not start as it should at that point. There may still be a conflict of sorts which is killing the program, but I believe it might just be a boot-sequencing problem, which one of your repliers--the one with the blue screen difficulties--alluded to. I would try changing the sequence first, and although I am hopeful for you, I am not totally convinced that the change will sustain a successful boot. If it does not, you have the information from David R to try, which looks like it has promise based on his previous experiences with the Avast system, which I am not overly familiar with. Long-time exposure has hopefully taught me to search for the simplest answers first, and observe, observe. I do wish you good fortune and hope I haven't led you down a primrose path with lacerating thorns alongside.