A bug in an update of Symantec's Norton anti-virus software caused some Windows PCs to crash late this week, making machines inoperable until they were serviced.

Symantec said that an update to its Symantec Endpoint Protection 12.1 and Norton anti-virus software for businesses caused some PCs running Microsoft Windows XP software to crash repeatedly.

Symantec said that about 300 corporate customers and about 60 consumer customers have been affected by the bug so far.

PCs could be fixed if customers manually removed the software from each disabled computer, the company said in an advisory.

Despite the relatively easy workaround, Symantec's customers complained on forums that the removal process was time consuming, although one said the software maker had offered compensation for the inconvenience - something Symantec has not officially confirmed.