NeoWin has posted a follow up on the troubles with Q811493 patch for Windows XP

You may or may not have seen an issue where after installing Q811493 the machine runs extremely slow. What we have determined so far is that it is an issue with the anti-virus on there machines. If the customer is running EZ Trust they will need to disable the file monitoring service and if they are running Norton they will need to disable the auto protect service and the installation of Q811493 and this will return to system functionality to normal.

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