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!