In my experience, the vast majority of XP infections have little to do with how up-to-date and patched the OS is. In most cases the infection is user-inflicted, sometimes made easier because of other software vulnerabilities, such as may be present in out-of-date browsers, Java, Flash, etc.
Since Windows updates are usually released monthly, for the next month at least, XP should be no more vulnerable than usual. It's only as time goes on that, theoretically, it will become more vulnerable, but it all depends on what new security vulnerabilities are discovered of course.
I've seen Windows XP machines, online and in daily use, running without updates and even without any service packs, that have no infections. I even had one customer that had several such machines - firewalls disabled, no AV and yet still no infections. On the other hand, I see fully patched and 'protected' machines badly infected frequently.