A repair/reinstall of Windows XP can be the last resort when you've got a really have a problem, like a major malware infestation. What I have found is you can't get the latest Windows Updates even after you've cleaned out the infection.
There are two ways to fix this problem. First, you can kick-start the file by re-registering it with the system. Launch a Command Prompt and enter these commands:
net stop wuauserv
net start wuauserv
That stops the Windows update service, registers the file with Windows, and restarts the service.
Second, you can download and install a small utility called the Windows Update Agent. Depending on how long this problem has been going on, you may have a lot of new updates to install once you've fixed it. Go to windowsupdate.com, click Custom Install, and accept all of the critical and security updates.