Press F12 at powerup.  Boot to the Dell diagnostics and run an extended hard drive test.  

What model system?  What OS?  

I had this issue due to me trying to be clever installing a dual boot OS without reading the instructions.  The only way I resolved it was by reinstalling W7.  Fortunately, I had backed up all the data a week previously since a reinstallation wiped out everything on the drive.  Back to normal now.