100% CPU usage

Hello,

I have a PC / AMD with Widows XP Home (OEM). Computer is 4 years old
and have not had problems until recently. Last December (2007) I
installed Norton 360.

Initially after installation I had no problems until two weeks ago. I
mention Norton 360 only becuase this is the only significate change
over the last year. I do not know if Norton 360 is the source of the
problem.

Booting the computer is not a problem but after I start to work the
mouse pointer freezes. I am able to cntrl alt del and bring up task
manager to see all programs running normally but CPU is at 100%.
After about 20 seconds the computer unfreezes. Usually I can continue
to work but sometimes all pointers have been lost and I cannot open
any programs. Compter shuts down normally and upon restart all is
normal until the next freeze.

During 100% CPU usage Process Explorere shows Explorer.exe and
ccsvchst.exe (Symantec Servive Network) each varying between 50 - 80%.
At the time I do not have windows explorer open.

Does anyone have an idea of what is happening and how to fix it?

Thank you.
.