A few days ago Microsoft released a critical patch known as MS08-067 (958644). If you have Automatic Updates turned on, and have it setup to install the patches for you automatically then you should have received this patch already. If you don't, I would suggest that you manually install it (download information).
This patch fixes a new security vulnerability in the Server Service that could allow remote code execution. For those of you who don't know what that mean, it basically means that someone can run a program on your computer without your knowledge.