If this is a recent issue, you might try using System Restore to take you
back to a time when it was working. Start\All Programs\Accessories\System
Tools\System Restore.
If that fails to resolve it, assuming you have an actual XP CD as opposed to
a manufacturer's supplied recovery disk or partition, you can try the
following:
Go to Start, type sfc /scannow in the run box and press enter. Note, there
is a space between sfc and the forward slash. You will be asked for your XP
CD. Be aware, upon inserting the CD the XP setup screen may appear, this is
not a part of sfc /scannow, rather it is being invoked by autorun. Simply
minimize the screen and allow sfc to continue.
If the above fails to resolve the issue, try a repair install as follows:
Be sure you are well backed up in case there is a problem from which you are
unable to recover. NOTE, while a repair install should leave your data
files intact, if something goes wrong during the repair install, you may be
forced to start over and do a clean install of XP. If you don't have your
data backed up, you would lose your data should that eventuality occur.
Boot from the CD. If your system is set to be able to boot from the CD, it
should detect the disk and give a brief message, during the boot up, if you
wish to boot from the CD press any key.
Once you have pressed a key, setup should begin. You will see a reference
asking if you need to load special drivers and another notice that if you
wish to begin the ASR (Automatic Recovery Console) depress F2. Just let
setup run past all of that. It will continue to load files and drivers.
Then it will bring you to a screen. Eventually, you will come to a screen
with the option to (1) setup Windows or (2) Repair Windows Installation
using the Recovery console.
The first option, to setup Windows is the one you want and requires you to
press enter. When asked, press F8 to accept the end user agreement. Setup
will then search for previous versions of Windows. Upon finding your
version, it will ask if you wish to Repair your current installation or
install fresh. Press R, that will run a repair installation. From there
on, follow the screens.
