I saw a message that an update was available for Microsoft Security Essentials, but it disappeared off the screen before I could click on the update button.I dug around looking for that message again and I think I found where I could click on the update button, by opening Microsoft Security Essentials.clicking on the "Upgrade now" button however brought up my ASUS Update program for updating my BIOS.

I logged off, then logged back in, and the "upgrade now" shows "Scan now". I don't think it upgraded yet though.

I clicked on the Help in the upper right corner of Microsoft Security Essentialsand there is an upgrade option in the list. Clicking on that seems to have brought up the proper upgrade application. I am now upgrading.

Glad to hear it is upgraded now. If there is any further concern about Microsoft Security Essentials, here is the contact support page.

I had exactly the same problem on my Asus EEE PC - clicking on the Upgrade now button in MSE always loads up the Asus BIOS update program instead. After reading this post however, I can confirm I also managed to upgrade MSE by the Help->Upgrade link instead. Thanks for the hint.