This month, Microsoft fixed a serious flaw in Windows and Office that allowed hackers to take over your computer using TIFF images. It also fixed a serious problem in the Windows XP kernal, along with patching other flaws in Internet Explorer.
If you have automatic updates turned on, your computer will install the Windows updates the next time you shut down. If the updates still need to be applied, you should see a yellow security notification on the red shut down button in your Start menu.
Click the button to turn off your computer and install the security updates. This could take a little bit of time, so make sure you don't need to use your computer in the mean time.