I have Windows Vista Home Premium.

I just updated to Service Pack 1.

Now my computer will not go into sleep mode. The screen goes dark, but it does not turn off, the screen comes back on as soon as you hit a button or move the mouse.

why?
how can I fix it?

Vista has alot of bugs that are not worked out yet.

All I can say is good luck! I just bought my computer in July and I have spent more hours trying to work through the bugs in Vista than I have any other computer in general. You might try contacting the computer manufacturer and see if they have any ideas.