As of April 14, 2009 the XP OS has finally and officially transitioned from the Mainstream Support phase to the Extended Support phase. Users will still get regular security updates and occasional hotfixes all the way until April 8th, 2014 and maybe longer if the demand is there.
The main difference between the two support packages is that Mainstream Support includes complimentary support and paid support as well as bug fixes for all those who purchase a retail copy.
Both types of support packages include free security updates. Basically all users will still receive security patches as Microsoft issues them Microsoft will also offer per-incident support via phone and Web for everyone. Customers who purchased a computer with XP on it will need to contact their OEM for support. Non-security related hotfixes will be available to businesses that are enrolled in the Extended Hotfix Support program.