BitDefender users started flooding the company's support forums on after a faulty antivirus update caused 64-bit Windows machines to stop working.
The company acknowledged the issue in a note explaining the problem. "Due to a recent update it is possible that BitDefender detects several Windows and BitDefender files as infected with Trojan.FakeAlert.5," the company said.
The acknowledgement came after BitDefender users had logged hundreds of posts on the topic. Some complained of being unable to reboot their systems.
"EVERY file that is trying to run is getting quarantined," one user, identified as lhmathys, reported. "Windows Explorer and even BitDefender update itself is being quarantined. Someone really screwed this one up."
"We are in a really terrible position now," wrote another user, identified as ufitec. "We have 150 business clients and most of the pcs [on] which BitDefender thinks everything is virus does not boot any more!!!!"
In its note on the issue BitDefender said it had issued a fix for the problem and offered instructions on how to repair the damage, saying that customers should remove files from quarantine and reboot.
Users who can't do that need to either use Windows' Last Known Good Configuration restore or System Restore options.
Earlier in the day, the company had advised users to disable the BitDefender antivirus module and wait for a new definition update, which is supposed to correct the problem.
BitDefender did not reply to several messages asking for comment.