Well it is a good thing you have a XP SP3 disc. Time to do a repair install of windows. If you follow the instructions, then what will happen is windows will be reinstalled, but all of your other programs and personal data will be retained. Some windows settings may be reset, but you can always change them again after.
There is a real good set of instructions with screenshots here
. Please look them over and ask any questions. When you are sure you are ready, go ahead and do the repair install.