"I just re-installed the operating system."
If by that you mean you ran a Repair Setup procedure, rather than installing your XP from scratch, if you have certain Windows Updates already installed on your hard drive, after you run a Repair Setup, Windows Update can download the Windows Update files to your computer no problem, but NONE of them will install, other than the first few.
Manually registering the windows update related *.dll files solves the problem.
(NOTE that this procedure can do no harm in any case.)
Go to
Start - Run
type: cmd (press Enter)
Type each of these lines one at a time, then press Enter.
The /s switch runs the commands "silently", such that no message is displayed.
If you like, you can leave out the /s and you will then get a message each time after you press Enter that it was successful.
regsvr32 /s wuapi.dll
regsvr32 /s wuaueng1.dll
regsvr32 /s wuaueng.dll
regsvr32 /s wucltui.dll
regsvr32 /s wups2.dll
regsvr32 /s wups.dll
regsvr32 /s wuweb.dll
To save yourself some typing, after you have typed the first line, you can use your right cursor key to display the last line typed again and just alter the last part of it.
When you have finished entering those lines, close the black window by clicking on the X top right.
You don't need to reboot.
Go to the Windows Update web page again, have it find your updates, and choose to install them again.
Windows Update will be able to install all your updates.
After a Repair Setup procedure, you have to re-install many of the updates you installed before, but some you don't - e.g. you won't need to re-install your .NetFramework version(s) and their update(s).