blkxxxxx.dat of each users isn't exactly same, so you simply can't copy one of them to your Bitcoin directory.
You must copy the entire folder from another user or sync from the beginning.
For sake of reference, check :
How to manually verify blk*.dat and rev*.dat files?Differing .dat file sizes on Pi node vs. Windows nodesIt's actually not because of randomness. People have been intentionally embedding viruses (or their signatures) in the blockchain so that antivirus software are triggered.
Unfortunately it cannot be recovered from quarantine or any other place from my PC.
I used Avast previously to help friends and all infected/dangerous files are quarantine by default.
Do you mean it's not available on quarantine list, which means it's above quarantine limit size?