I don't even think you need a screwdriver to open the mac case ;-)
Yes, you need a medium Phillips screwdriver.
As for the loud fan noises? It's quite common on the iMac (I've had it happen on my G5), but I don't think it's something to worry about too much. Basically, Mac OS X controls how much fan to turn on based on the temperature. If something goes haywire, such as Mac OS X freezing or something, it can no longer evaluate and control the fans. This is when the emergency backup kicks in, so to speak, it powers full blast to make sure that nothing overheats. So although it sounds scary, don't fret it. (And the fan noises always get loud in Single User Mode, too.)
To fix the starting up, let it sit there for about 20 minutes. If it still refuses to start up, I would suggest booting from the OS X install disk, and then doing a Archive+Install.
Reputation Points: 2240
Solved Threads: 338
Vampirical Lurker
Offline 5,055 posts
since Apr 2006