"last known good configuration" can sometimes fix this, but not all that often (there's numerous causes for this error). worth a try though cos it's by far the simplest of the potential solutions.
failing that, i'm afraid that unless you've got the Windows Recovery Console already installed then you're going to need the installation media (XP install disk) in order to have any chance of repairing this. unless you can get your hands on a disk with EXACTLY the same version of Windows that's installed on the machine, then yr computer shop is probably your only option...
if all this fails, then you've got a failed hard drive and it'll need replacing.