You can boot from your Windows XP CD and do a repair on Windows, however I suggest using a SP3 edition so you dont have to do as many updates.
First restart your PC with the Windows XP CD in the Drive (if your PC is set to boot from CD before the Hard Drive, you will get a prompt to Boot from the CD "Hit any Key"... If you dont see this and you see the Windows screen loading, the Boot order in the Bios will have to be changed.
After you boot into the CD, it will load some temp files and then the first thing it will ask is if you want to setup windows or to enter the repair console. You want to choose the Windows Setup (if I remember right its the top option of the two).
Then you will have to hit "F8" to agree to the Lic.
Then it will scan your Hard Drive automaticly for installations of Windows, when it does see it, it will come to another screen asking if you want to run a Repair on it, DO THAT.
The PC will start to copy temp files to the Hard Drive for the next part of the repair and will restart your PC when its done.
Let it boot like normal (not from the CD) and it will go into the second part of the repair.
About the middle way through the setup it may ask you for the Product Key for Windows so have that handy just in case.
When it comes up for the option to Make a User, click on "Skip" button on the bottom right of the screen.
Then log into Windows like normal. That will fix the missing files. Remember to check Windows Updates after the Repair is done.