Ok ive read around on various forums, each seems to have a different way to fix this issue, however i know a few of you are working in Support etc,
so you must have come across this issue recently.

How do i add further updates to XP?

Just installed SP3 recently and cant install todays updates.

I had the same problem. It was down to a few things.

invalid root certs/corrupt cert db.
unregistered dlls

When you fail to install updates on the web update itll tell you some articles to look at. look at them, theyll solve it.

I was tempted to uninstall SP3 and try and update the smaller updates first before a reinstall of SP3, think this will work

Hehe nevermind, ill do it the other way ty

Its a known issue that also happend with SP2.  

1. Stop the Automatic Updates service. To do this, follow these steps:

a. Click Start, click Run, type cmd, and then click OK.

b. At the command prompt, type the following command, and then press ENTER:

net stop wuauserv

2. Register the Wups2.dll file. To do this, follow these steps:

a. At the command prompt, type the following command, and then press ENTER:

regsvr32 %windir%\system32\wups2.dll

Note For a computer that is running Windows XP Professional x64 Edition, type the following command, and then press ENTER:

regsvr32 %windir%\syswow64\wups2.dll

b. Click OK on each verification message that you receive.

3. Start the Automatic Updates service. To do this, type the following command at the command prompt, and then press ENTER:

net start wuauserv

Your Finished, Now Update.

Fixed this already, and not that way