[dba-Tech] WinXP update problem

after running an update on WinXP pro SP3 I get following message:

Some files have been modified and need to be replaced. please insert the 
Windows XP SP3 CD.

When I do that, it tells me thats the wrong CD.

Of course it is the correct one. it is the one from which I installed SP3.
Still, I burned a new CD from the ISO file which I still have.
no success. still 'the wrong CD'

This happened on two PCs
I restored system files to a day before and the PCs are working, but I have 
of course turned AutoUpdate off.

Any idea?

thanks for any proposals what to do.

Lembit