ESET NOD32 Anti-Virus 5 in Safe Mode?

I have Windows 7 Ultimate and my computer is protected by Nod 32 Anti Virus 5. It's a brand new version and its activated with a six month premium license. I wanted to boot it in safe mode the other day and it opened up as a DOS window. I let it run for about 30 minutes to an hour, as I wasn't keeping track and then the DOS window closed. What just happened??