You can use this method as often as you like when you need to repair something that had gotten corrupted and you don't want to do a clean install.
This is better than doing a windows repair install from the cd because doing that can cause changes to settings and even make it so you can't get updates anymore from microsoft update site because of the 'silent install' they did a year or so ago (unless you rereg some dll files).
1- Start WindowsXP
2- Go to the location of your source files
3- Run WINNT32 /unattend
For example, D:\I386\winnt32 /unattend ( copy & paste this command into the RUN box, then press ENTER ) Your computer will do the rest.
OR
Start > All Programs > Accessories > Command Prompt.Copy & paste > D:\I386\winnt32 /unattend (notice the space between 32 and / )Press Enter.
If you have i386 on your drive, you can replace D: ( if D: is your cd drive ) with the location of it. (be sure it's the full i386 though)
D: is the driver letter where your windows XP Cd is if D:\I386\winnt32 /unattend doesn't work then try : d:\winnt32 /unattend.