I’ve already run NRT after uninstalling NIS. Twice , and rebooted in between.
Then I installed AIS 5.0 and XP ran fine.

I’ve run it though, as per your suggestion.
Also, uninstalled AIS,rebooted, ran the avast uninstaller but as of now XP has become completely sluggish.
I’ve reinstalled AIS 6.0 but XP remains sluggish.
No improvement whatsoever. I’ve tried repairing AIS from Config->Software->AIS but I get an ‘Error processing packages. Please use full update’ Avast popup.
From sandbox issues and multiple frozen FF sessions, I’ve gone to a seemingly borked XP system.

I’ll now try to restore to a previous ‘system restore point’ but as you’ll perhaps understand, AIS 6.0 proves to be a nightmare.