Microsoft has issued an additional patch for Windows XP Service Pack 2, to stop the system crashing as a result of users installing unnamed third party anti-virus software or firewalls.
Microsoft said the patch was not a security fix but an operational solution to fix what it describes as a "critical" flaw. The company now usually issues batches of security fixes on the second Tuesday of every month.
Users affected by the problem will see an "invalid process" message when their system crashes.
The latest patch also clears up problems that affect Windows Server 2003 and the Windows XP Tablet PC Edition 2005. The patch is available from the Windows Update part of the Microsoft website and is being included in the Automatic Update service.
Microsoft is currently involved in a major roll-out of its own security software products, and analyst Gartner expects Microsoft to have its own combined anti-spyware/anti-virus solution later this year.