Computer in a snit again.

Computer has been acting good lately until the past few weeks when it got its knickers in a bind! All I was trying to do was to improve its performance by doing what netsquirrel.com says one should do weekly. I checked for new updates, next I did the scan disc thing, I did what it said, and it took its time, you know where its says, 15 % done, 2% done, etc? Evidently the machine thought that was jolly fun because then it would erase, and start it over again, AND again, AND again, over and over. So I got up and left for an hour, came back and silly machine was still going at it!
So I stopped that and went on to the next 'helpful thing', defragging. I did the proper thing (pictures show what to do), and sat back when the defragment began. This time we advanced to about 6%. It went back and slowly crept up to 6% done, erased and did it again and again! That when I told it to enjoy itself and went to bed, got up a few times, it was still going!
What I need to know, before I make good on my threat to replace the machine with one who will work happily in harmony with me, is there anything humanly possible I can do? I'm sure it needs the defragging done as its been months since last done. Any ideas that are simple enough for me to understand? Or should I call Earthlink? or Compaq? Thanks much, Doris