HELP! iMac is stuck on the white screen?

This is how it looks like once I turned it on: http://www.youtube.com/watch?v=lpiBgXQCJ4k&feature=related

What should I do? I was able to log in yesterday after doing the PRAM method and now this method doesn't work. Should I do what's suggested in the top comments? It's alredy been 4 days since this problem persisted.