Windows Vista Freezing system

Hello ,

by a couple of weeks i have some issues with random system freezing and sometimes blu screen crash.
My pc is a laptop alienware with cpu: T7200 - 2ghz, ram=4GB, vga=2xgeforce8700gt 512mb, system windows vista.Drivers are updated.
I made antivirus scanns , found some and removed but seems that problem is not solved as i had freezing again.
Made a memtest run by 2 hour with cpu usage and ram usage at 100% and had no errors or other issues.So i think it's not a hardware problem
The error i find is :
log.name: system
source: eventlog
event id: 6008 (qualifiers 32768)
level : error
level 2
task category: none
'' the previous system shutdown was unexpected''

Here is the log from hijackthis if can help:
Logfile of Trend Micro HijackThis v2.0.2

[Admin: Malware logs are only allowed in the Malware removal forum]