xp crashes and reboots shortly after installing norton antivirus

Hi-

 I've been having some problems with my hp desktop since I've installed norton antivirus 2010. I had some problems with another antivirus and ended up taking it off and then took it to a repair shop. It was working fine for several months and I know it was virus clean because they checked it and I didn't put it on line.. I bought a laptop for those needs and was using the desktop for writing, thinking I would install the norton for it from a disc and then update all the windows patches. It has only been online for updates from m.s.; which I install when ready, and norton; which is automatic. After a few times of shutting it off and on it began to crash while I was writing. Sometimes with little glitches where it didn't actually go away it just blinked. After the first windows update it crashed during the install. It did this several times before I finally got it to work then went to the ms site and checked for updates and reinstalled. When it started acting up I bought a tb hard drive and backed up the files. I bought a new cooler unit (upgrading to  copper) and fan just in case it was overheating. The thermal paste was on it. It seemed to stay on longer but it still crashed and rebooted. Time for more ms updates. Acted just like before. Finally got them on they same as before. It is usually scanning when  it crashes the rest of the time. This last time it crashed it tried to reboot and crashed as soon as the desktop came on. It will just keep that up until it's unplugged while it's down. I got the blue screen once and set it to come back on at the last known configuration and it repeated the last thing. The last time it went straight to a does screen with system setup warning listing. Cache ram is 256kb,  intel pentium 4 chip and speed is 1900 mhz/400mhz. If you need it It has also been slow to startup. Please help me get this thing on and lined out.

fox