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Hi ady4um,
Yes I did read the previous post and cleaned up Avast using the tools. I could see versions 4 and 5 had been previously installed and removed. Even in registry I could see references to v4&v5.
My estimates were wrong and their were actually 9 digit files in the folder! Fortunately it only took about 4hours to clear the folder at a command prompt. A chkdsk /f on reboot reported no errors, avast cleanup tools ran, and for the next few days I have installed Microsoft Security Essentials for low level security while I research what may cause this.
I suspect its a combination of something happening, or a corruption or crash that may cause windows to auto-correct a corrupt file which ends up being fragments. I doubt I’ll ever be able to reproduce the error, but an interesting one indeed.
At least disk cleanup works for the owner again, as it was getting stuck in that folder 
The other tool I notice installed is Malwarebytes Anti-Malware - installed as the free ‘no live scanning’ option. Not sure if this would cause this issue, I havent seen this outcome with any other avast install’s Ive seen.
Will see how we go over the next week.