This same thing happened to me on two different machines when I upgraded McAfee to 2009. Defeneder can not run at the same time as McAfee.
Run an Adminstator DOS prompt by right clicking it from the Accesories start menu. Then type in the following or cut and paste.
sc config WinDefend start= "disabled"
sc stop WinDefend
You must press Enter after each line if you type it.
Enter only one line at a time.
Hope this helps.