Microsoft has posted today a critical fix to Windows XP Service Pack 2 (SP2) on its Windows Update site, and pushed it to users relying on Automatic Update.
The goal of this update is to address a problem (public since December 2004 and that affects also Windows Server 2003 and Windows XP Tablet PC Edition 2005) where, in some cases, the installation of third-party antivirus or firewall software could bring down the operating system (the famous “Blue Screen of Death” with a cryptic error that reads "Stop 0x05 (INVALID_PROCESS_ATTACH_ATTEMPT)."
The upgrade is recommended (and this is one of the cases where problems are not on the OS itself, but caused by third party software).