Computer Shutting Down

Running Windows XP Home. Norton Internet Security told me that it detected two intrusion attempts this morning and ever since, my computer has been shutting down at will. Shut down is automatic (does not give any warning, it is like turning off the lights). Have looked at symantec and mcafee sites but found nothing useful. This has been happening more frequently while ripping songs into iTunes, but will do it when just surfing the infobaun as well. I am on a wireless network if that makes any difference.