XP Fatal Errors-Shut down?

I dont know where to begin? This past month has been a night mare, of XP could ever have. It started with the change, from Norton 2006 to update 2007! I know that I save alot on my puter but, I do clean the history and all the web pages I view, upon leaving, the web. I was in the process of doing some printing for my sons business (because I am a good mom) and its cheaper if I foot the bill now. My puter stopped functioning? I have a gateway pentium 4- 1.3 with 512- no more than half of the disc space is used, I check it alot. Well back to the mess, I purchased the up-graded version of Nortons-2007 Internet and the Basic edition for 2007. I tried to load the Basic Edition several times- it wanted to take the "Norton go back off my puter" I told it NO. It stoped loading, and just sat there. I removed the disc and re-installed 2006 version- then loaded the 2007 Internet Version. I asked if this would hurt-(Nortons) no, so I have left it as is for now. Since then I have had 4 major FATAL Errors show up The system has recoverd- dont ask I have no Idea, but it knows what is causing this problems, how do I find out? I have run several other programs- PC check- Nortons and everything seems fine, defraged and deleted alot of unused files? How do I find what is causing this lock up?