You might have heard about a nasty vulnerability in Internet Explorer that allows a malicous website to remotely take-over one's machine. Microsoft just announced a fix for this issue that will be released tomorrow. It should be available via automatic updates, but just in case I'll follow up tomorrow with links.
More info about the vulnerability:
If you previously applied any of the complex workarounds for this problem, you will need to reverse your changes before applying tomorrows update.