Diagnostic logs are produced when you download and run the first three programs. Available at this link: https://forum.avast.com/index.php?topic=53253.0

The way it will work is if you run these three programs, they each will produce a log. Attach these logs in your next reply. A certified malware expert will examine them and propose a fix specifically for your 8.1 system; you run that fix and report back soon after running them.

Since your 8.1 system seems crippled, suggest using an USB flash drive to transfer programs and logs back and forth as necessary.

You will need this program installed first in your XP system to prevent any usb infections when moving files to and from your 8.1 system: http://www.mcshield.net/