On April 26th Microsoft released a security advisory that affects all supported versions of Internet Explorer. This includes versions 6 through 11. So if you are running windows and use Internet Explorer this affects you.
The vulnerability could allow an unauthorized person to gain access to your computer.
Today, Thursday May 1st 2014 a patch was pushed out to fix this issue. Even though Windows XP is no longer supported by Microsoft, a patch was released for this version of windows also.
What should you do to keep my PC secure
If you have automatic updates turned on, you will get the update pushed to your windows PC automatically.
Until you get the update, the Department of homeland security suggest you use a browser other than Internet Explorer such as Chrome or Firefox.