I had a similar problem about a month ago, that started off with the system booting up then either locking up, or having the screen go blank then lock up.
I managed to ignore it for a while, then it got progressively worse, to the point where I couldn't get it to boot up, without hitting 'F8' and doing a basic boot.
Long and horrible story short, I found a web site that instructed me on how to go about ripping every little 'bit' of Norton and Symantec programs out of my system.
The system purrs like a kitten now.
If you're still having problems check out this site:
"How the $#@$# do I fully remove Norton from my PC?": Tech Support from Ask Dave Taylor!