System File Checker (SFC) does not work

Hi,

I have a Dimension 2400 with XP Home edition Sp2.  A few weeks ago the computer switched back to SP1 by itself. Nobody else but me has access to my home computer. The only thing I can think caused this problem was having two antivirus programs loaded at the same time for over a month. I reloaded SP2 and have been having problems with things not working. I thought that by using the System File Checker I would be able to fix the missing or corrupted files but I cannot get it to work. When I go to RUN and type the command SFC, or SFC\scannow the command screen flashes for a second and nothing happens. I have read all of the past postings ( very old ones) in this Forum about SFC but none of them really relate to my problem. Most people had problems with the SFC because they were asked to use the Windows Cd. Mine does not even come up at all. I tried using the Windows SP2 Cd  like suggested in one of the posts and doing the command but the same thing happened. I searched my i386 file and tried to open SFC.exe and could not do it. Is there a way to fix the System File Checker? All of your help would be greatly appreciated.

Thanks, Amy