vista wont stay sleeping

I'm not sure if this is a common problem, but I couldn't find anything on it.. Basically, when my computer goes into sleep mode, the screen immidaately turns black, it stays like this for about 10-20 seconds, and the fan on the computer doesn't turn off. There is a click, when my computer goes into normal sleep mode, for about half a second and then it wakes up again with the normal "Windows is resuming from sleep..." message.
I think its probably a program that keeps waking it up, and I know that I'm not pressing buttons or anything... But I can't figure out what program it is or whatnot.. please help me!