Avast failed me, after installing avast on my windows 10 setup yesterday I was forced to roll back remove and go back to using Windows Defender, however I dont think it was the Anti-Virus product, I think the problem was caused by that Optimization / Clean up wizard tool they throw at you for £14.99 when you get there free anti-virus package, I know you don't have to buy into that extra add-on but hey it seemed like a good at the time.
Anyway after installing the latest version of Avast 2015 the other day and opting to dish out £14.99 for there optimization clean up tool, the installation was successful, I ran the Anti-Virus not issues where detected, been using Defender since Windows 10 Release so according to Avast that never fail me, then was given the "we can fix 30+ or whatever performance / registry issues we have found if you buy our clean up optimization wizard at the discounted price of £14.99" so I figured ok why not I have used Free Avast before on numerous systems with just about everything since Windows XP at least, 7, 8.1, never had any issues with them before so fine I trust this software they have been providing me with a free service for years, never had then on a Windows 10 set-up before so what the hell I figured I would give there clean up wizard a go, I let it clean my Reg the lot.
Afterwards PC would simply not start, every time I got as far as loading the first sign of the Desktop in Windows "Critical Error Message, stating something along the lines of Cortana and Start Menu are not working we need to restart your system and try and fix the problem next time you log in". but no joy on fixing the problem turned out to be a continuous loop every time I tried logged in only to be kicked straight back out.
The fix was no real big problem I guess, just had to run a system restore back to just before I installed Avast, so went ahead restored and uninstalled Avast Anti-Virus, Clean Up tool the lot, turned Defender back on and everything once again is fine, and least Avast told me one thing when it found no issues on the Virus Scan, that Defender was doing all right.
I would have always recommended Avast as my personal No.1 Free AV choice for as far back as I can remember, but after having experience with it only yesterday I simply can't this time round, least not on a Windows 10 system, my advise stick to using Defender, and if you do choose to try Avast beware of the Clean up tool that they want to sell you with it, and at least if you do want to give it a try, insure you have a good System Restore point set up just before hand.