Outlook 2003: Explorer 100% CPU???

Recently, I have been facing a very frustrating problem.
I am using my Outlook 2003 for getting my Yahoo mails via POP3.
However, recently, every time my outlook (automatically) sends and receives, it does freeze my computer. It takes a very long time to get very small mails (several seconds and even minutes) and during that time. my computer is frozen. Then, when the e-mail has arrived (icon gone), things go back to normal.
I have checked the process and explorer runs at 100% which leads me to think about a problem with XP.
I use Windows XP, Office 2003 and Norton Internet Security 2007.
Thanks a lot for your help.

Iwan

Note: this is even worse when I am on battery and my CPU is set to slow to save battery. It takes a huge time…