hl2.exe set off when a shotgun is used on a NPC

Plz help iv tried everything to get this to work.I even emailed garry and his support email and recieved no reply so i need help.when i use any shotgun on any NPC it crashes and i get the hl2.exe message.every other weapon is fine and this is a real problem for me as all my friends like to come round to my house and play gmod and plus its a pain in the ass.Plz help

Post your system specs, along with what source games you have.

i have counter strike source,the orange box,and Hl2 deathmatch and the latest gmod
i dont really know what specs are so il just list basic stuff
Laptop name- Dell inspiron 1525
proccesor type - Intel core 2 duo T5800
Processor speed 2GHz, 800MHz FSB, 3MB
Memory Size 3 GB MB
Memory Type DDR 2
Hard Drive Capacity 320 GB
Graphics Card Type Intel Media Accelerator X3100

anyway i dont think my specs should be a problem my friend has exactly the same kind of laptop and gmod works perfect on his but it may be stuff tht i might have got from joining servers so il try reinstalling gmod
whenever i start a game in gmod i get this message
entities/immolate/init.lua5: attempt to index global effect (a nil value)
also when i was screwing around i found tht if i was shot it would crash and tht if i shoot hunters with anything it crashes.