As it turns out - the culprit was Norton 360 - the driver IDSvia64.sys caused the system to constantly crash.
What is weird is that I've been running windows 7 64bit for a long time and for just as long has had Norton 360 istalled and have never experienced this problem until recently.
In any case once Norton 360 was completely uninstalled and the registry was cleaned of all nodes and keys relating to norton and sysmantec the PC is now running smothly and no more crashes
For antivirus protection I am now running Microsoft Security Essentials