Ok, I found a quick fix for the cultist las-pistol bug in XP. however, you will lose some of the challenge in the game.
Go into the "Data" file in the chaos gate directory (you may want to back up this file first)
Open the "WeapDef" file using wordpad
Scroll down to the las-pistol values
Modify the amount of Ap it takes to use the pistol from 15 to 50
"Save as" and replace the old WeapDef file
The Chaos cultists should no longer be able to use their las-pistols, preventing the bug from occuring.