My father's Desktop, a Lenovo (Core i5 6GB RAM) is having some issues and I am not sure what to try next. After a fresh start/reboot, after 3 to 5 min, it begins slowing down and barely reacting to mouse or keyboard commands and within a minute or two, has no reaction to commands.

I've run HDD diagnostics on both the boot drive (Seagate 500GB) and on the data drive (WD 1TB) and both extended tests passed. I ran memtest on the RAM (4GB & 2GB dimms) overnight and they passed as well. I unplugged all components except boot drive, fans, 4GB RAM, and CDROM and disabled all Non-MS services and disabled ALL startup items. Same issue.

He is running AVAST free and full version of Malwarebytes. But, I'm not able to even begin any kind of scan with mbam.

Event Viewer ... nothing jumps out at me in windows log files as to what the issue may be.

What should I try next?

Many thanks!