From Opera's changelog

• Fixed an issue where specially crafted JPEG images ccould be used to execute arbitrary code, as reported by Tavis Ormandy of the Google Security Team; see our advisory
• Fixed an issue where plug-ins could be used to allow cross domain scripting, as reported by Adam Barth; details will be disclosed at a later date.
• Fixed a moderately severe issue; details will be disclosed at a later date.
• Added support for the following platform-specific features: 
  • DEP (Data Execution Prevention) in Microsoft WindowsXP® with Service Pack 2 and higher and Microsoft Windows Server 2003® with Service Pack 1
  • ASLR (Address Space Layout Randomization) in Microsoft Windows Vista®

Read more: http://www.opera.com/docs/changelogs/windows/964/