sleep mode not working

I have been trying for two days to get the sleep mode to work with my system. I am running a custom built desktop. My monitor will power off but the harddrive light will flash every few seconds and all of the case fans are running. I am 99% sure that the sleep mode was working before I setup media center. Once I set it up is when I noticed the sleep mode stop working. I have messed with all of the advanced power settings with no luck. Any ideas? Any help will be greatly appreciated. Thanks