I should have posted earlier - I have tried but made the setup worse.
I am trying to sort out an out of date Windows XP on an Acer D250 I wanted to pass on to the children.
It had got some dodgy toolbars loading, so ran Malawarebytes Anti-Malware (up to date), and it removed 20-odd items.
Meanwhile XP automatically updated (quite a few updates). And I installed Microsoft Essentials, and ran that through too.
Now Windows Update would not run - it just hung with the moving bar.
So ran SFC /scannow - if found some errors, which it couldn't fix, as it's a netbook without a CD.
Changed the entry in CurrentVersion\SetUp\SourcePath to C:\ [from C:\windows\system32]
Ran SFC /scannow again, and it ran through (and was rebooted).
Now Windows update (after a wait) states it needs to download "Windows Genuine Advantage Validation Tool", which fails with 0x80246008, and all the previous updates are all marked as failed (same error).
And Essentials gives error 0x80240022, unable to update defintions.
The internet connection works fine otherwise.
I did try the update 'FixIt' but it failed on three counts (registration missing and 2 of installing recent updates).
Another reboot,and still the same problem.
Any pointers would be appreciated.