BitDefender has apologised to users affected by a faulty antivirus update and updated its customer support pages to offer a fix for machines that have stopped working.
The security firm confirmed a recent update was faulty and had caused 64bit Windows to stop working.
"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 on its website.
The acknowledgement came after BitDefender users logged hundreds of posts on the company's support forums 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 very sorry for the problems people may have had in regards to this issue," Matt Kicks fromBitDefender said.
The security firm has issued a patch and its customer support pages offer detailed information on installing the fix.
Home users of BitDefender 2010 and BitDefender 2009 should follow these instructions, while BitDefender Business Client users should use this solution and BitDefender Security for File Servers users must follow these instructions.
Kicks said the support articles are being updated as soon as new solutions are implemented.
See also: BitDefender tool removes Conficker worm