Think about any changes you made in drivers, software, or hardware just before the problem started. You could also check the rather cryptic system event log. To do that click Start -> Control Panel -> Administrative Tools -> Event Viewer. Once in Event Viewer click on the system log and scroll through the entries looking for clues to see if you can find hints as to where the problem could be. Be aware that this log is very cryptic and it's a bit like looking for a needle in a haystack.
A few months ago my computer stopped going to sleep via the timer settings. I had it set to sleep after one hour of inactivity. I can still force it to hibernate or sleep via the "start" menu. In addition, I have a floppy drive (not being used obviously) that seems to get accessed every once in a while, I don't know if this is connected to the problem. I'm assuming that some process is setting/resetting the sleep timer but I don't know how to find that out.
Any help would be greatly appreciated.