Windows XP support officially ends tomorrow, as does Office 2003. The last patch Tuesday for Windows XP is going to be a small one, though. So, after Tuesday, you can update your Windows XP machine and be done. It's probably best to update to Windows 7 or 8, though.
There will be a total of four updates released for all products, two for Windows and two for Office. Only one of the updates for each product is rated critical, although we don't yet know the number of vulnerabilities addressed for any of the products or their exact nature. All four updates are for remote code execution vulnerabilities.