Computer won't start after shutting down os x

[pet1]

This just started happening a few days ago. I noticed that when I shut down os x for more than about a few minutes, my computer won't start up. It turns on, but the keyboard and mouse will not light up. I have to reset the cmos to get it working again. This only happens after I shut down os x. It doesn't happen when shutting down windows. If I just restart the computer it's fine. Or, if I shut down and reboot a few seconds later it's fine. This happens when I shut down os x only for more than about 5 minutes. Using 10.4.5 on intel 915g with pentium 4 630. Never had this problem before.