Help me with my gmod crash problem please!.

when i go to a server everythings fine but when i get shot or something or somthing blow up it lagges like hell then does the hl2.exp thing has stoped woring how do i fix that? on rp servers when i take like drugs it crashes to and like every 5 mins it crashes.

PC specs?

System Information

Time of this report: 1/2/2010, 08:56:45
Machine name: HP-PC
Operating System: Windows Vista™ Home Premium (6.0, Build 6002) Service Pack 2 (6002.vistasp2_gdr.090803-2339)
Language: English (Regional Setting: English)
System Manufacturer: HP-Pavilion
System Model: FK543AA-ABA a6667c
BIOS: BIOS Date: 09/05/08 10:24:25 Ver: 5.30
Processor: Pentium® Dual-Core CPU E5200 @ 2.50GHz (2 CPUs), ~2.5GHz
Memory: 4094MB RAM
Page File: 3863MB used, 4538MB available
Windows Dir: C:\Windows
DirectX Version: DirectX 11

i think thats what your talking about mabey