iMac stuck on blue screen on startup

Try this:

You will need to type some Unix commands. If you are not comfortable with this, I don't know of anything other than a re-install. But if you are careful, you should be OK. I recommend you print this out in a largish mono-spaced font so you don't miss any spaces (or add extra ones). Note that case is important.

Be careful. Some of these commands are dangerous, since you are going to be root.

Start up in Single-User mode by restarting the computer. After the chime press and hold down the COMMAND-S keys until you see white text on a black background. When this has finished you will see a prompt ending in '#', although there may be other messages. Enter the following commands after the prompt:

/sbin/fsck -fy

Press RETURN. Wait a few seconds for 8-10 lines of output. If the last line says repairs were carried out, repeat this command until you get a message 'The volume <yourdiskname> appears to be OK'. Then continue with:

/sbin/mount -uw /
cd /Library/Preferences
rm com.apple.loginwindow.plist
rm com.apple.windowserver.plist
cd /Library/Caches
rm -r *
cd /System/Library
cd /System/Library/Caches
rm -r *
reboot

Press RETURN after each command.

This should now take you to a proper login screen after the normal boot sequence. You should then Repair Permissions by using Disk Utility (in your /Applications/Utilities folder).