Excessive CPU Usage Dimension 4100

A good friend has a Dimension 4100 she bought new a few years ago. The computer has remained in completely stock condition. That is, there have been no modifucations made, no hardware upgrades or component replacement of any kind to this computer and is 100% OEM, including the Windows ME. She has also been the sole user of the computer since day 1. This computer has run like a champ until the CPU problem started and has given her practically zero problems since new. This computer has run so nice that one would have thought it was still a brand new Dimension 4100. Then without warning excessive CPU Usage. For a while it was somewhere around 64 - 66% constantly with sudden spikes to 100% every 20-30 seconds or so. Not long ago it started running 100% and is now a constant 100%. It also has starting to shut down every so often on her and is sometimes difficult to get booted up again. I have checked for spyware, viruses, everything with no results. I have also checked to make sure nothing was running that shouldn't be that would cause excessive CPU Usage and have found nothing. Can anyone help me with this bizarre problem before I decide it's time for some water cooling by dumping it into the river?