There are usually a few repairs for this but first we discuss saving the files we can't lose. I suggest the cheap fast save is to put in a new drive (40 to 99 bucks), install the os and copy from the old failed OS to the new drive and to CD/DVDR media.

Now that all is safe we can boot safe mode and use the System Restore to go back. If not we boot the XP CD and attempt a Repair (from it's menu.)

Bob