I know this sound obvious but did you check the connections?
Once my heat in my house went out over for about week. During that time I tried to boot up my computer and it was doing almost exactly what you've described here. After trying about 100 different things, I finally checked the connections. What I found was that IDE cable connection was lose around the controller (on the motherboard) and it it very lose around my slave drive. After I pushed them all back in snugly it all worked fine.
Now, Since I'm the only one that lives in my house the only way I could imagine this is happening is that it got cold enough to contract cables and the connectors enough mess with the connection.
I know it all sounds far fetched but the computer is a old p90 (even had the error chip) and it was the only time I had that type of problem with it in the 6 years I've owned it.
Hope it helps,