Microsoft has released an Out-of-Band (read as not released on the usual 2nd Tuesday of a month) update to address some critical security issues.



If your computer is set to automatically update, then no worries, otherwise, have Windows Update check for new updates.

Here's a good writeup and explanation:
Microsoft releases out-of-band security update to plug .NET hole | ZDNet

And always, if Windows Update tells you that you are uptodate, don't you believe it!

Click on "Check for Updates" in the left hand column and guess what--Windows Update will come back some updates. They may call them important but in reality they are critical.