I haven't looked much into what is causing it as to more looked into a solution, although i don't recommend using XP online anymore (even if you have posReady updates) I can give a simple solution for a Manual Virus scan of the XP.
First Download the MSE update from here and then just set your windows Clock back 1 Year, then double click the Update and it'll install, MSE will go green and you can scan :)
Any other problems, go into Control Panel, Administration Tools, Services, Highlight Microsoft Security Client, hit properties set to automatic then click Start and apply.. But the DATE is important as the 126.96.36.199 version of the APPs life cycle has Expired now folks.
|Latest Pattern File October 2016, System Date Set Back 2015 Scan Now Possible|
for some off reason that lads tool "Fix MSE 1.0" wont run on SP3, says MSE not installed so i had to manually restart the service after i had dumped this code into a BAT file to do the dll's registration.