Well, may be you want to check your memory ram, in some cases, sometimes, the memory ram goes bad and it causes a terrible comportament in the personal computer, I think that you should check your memory ram. There is a lot of programs to do that just search, find, and install it, test and see what happends. Also if you want to be more secure, you can go to an electrical shop center, and here, bring your memory ram, to a check, and they will say to you if your memory is fine or is going bad. Try it you do not lose nothing. I think this could be the problem

Try to take a look on what is running in the background using task manager....
Norton apps dont mix up well with other firewall/Antivirus programs....
Use Karpersky Antivirus Pro and Look'n'Stop firewall, it will save you a great deal of memory and make your computer work faster...
Try to keep your windows updated, as well as your NVIDIA drivers.... a wrong or old graphics driver can cause those crashes... just a thought...

Remove MSN Messenger, use GAIM... its free and problem free...

dont use Internet explorer or Outlook express.... try to use Mozilla apps like Firefox and Thunderbird.... their better, and a lot safer...
Also, Kasperky AV PRo may clean your pc better from infections and threats...

Hope this has helped you



Sk