I just started having the same problem, my PC screen goes off but my fan are still running. The funny thing is that when I boot under XP, that doesn't happen.
I looked into my device manager and saw one thing that was not OK (can't remember the name, I'm at work). Something like Torenza tunneling... bla,bla,bla. I'll probably try tonight to remove it and scan for new hardware to reinstall it. It says that a key in the register is corrupted and that it doesn't work for that reason. Remind tough that this may have nothing to do with my problem after all.