Thanks ObiJuan,2080, unfortunately it didn't work for me. For anyone who tried ObiJuan2080's solution and it didn't work, I still don't know what the problem is, but I did find a temporary way around whatever's going on.
When I boot up the computer, as soon as I hear the "bommm" tone that sounds on startup, I press down my eject key. And I hold it down until my laptop's totally up and running. I have to do this every time I boot up my computer--f I shut my computer down, I have to do it again, but it's easy enough.
I have no clue why this works, but it does work for me...a small fix for a problem that still mystifies me.