Try creating a new administrator account and see if you still have the same problem. If you dont , delete the other accounts and recreate them. Just eb sure to back up the documents folders of the other users before deleting them

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Alternatively you can try downloading the trial version of Spysweeper from our website and update it and then do a scan to see if it is a nasty bug causing this. here is the link
www.plug-n-playpc.com . the link is on the right hand side of the page.