windows 7 - Blue screen - BCCode: 7F

I've recently bought a refurbished Toshiba Satellite A660 11M and I get the blue screen of death appearing fairly frequently now, and I can't seem to work out why or how to prevent/cure it.
Firstly I thought it was due to Zone Alarm from what I read on the net, but then I thought it was a memory issue after it crashed whilst I was running Memtest. But after I ran Memtest a second time, it completed without any errors...

Here are my computer specs -

Toshiba Satellite A660 11M
500 GB hard disk
4 GB RAM
Windows 7 Premium
Nvidia GeForce GT 330M

and here is more info regarding the crashes I keep getting -

Problem signature: Problem Event Name: BlueScreen
  OS Version: 6.1.7600.2.0.0.768.3
  Locale ID: 2057
Additional information about the problem:
  BCCode: 7f
  BCP1: 0000000000000008
  BCP2: 0000000080050033
  BCP3: 00000000000006F8
  BCP4: FFFFF80003040EC0
  OS Version: 6_1_7600
  Service Pack: 0_0
  Product: 768_1