I have a Dell optiplex gx270. It has been working fine until now. It started freezing up on me and then when I rebooted I got message "press F1 to continue or F2 to enter setup". I went into setup and the hard drive did not show up. It said unknown device for primary IDE device. After shutting down for a while I tried again and same thing. Finally I was able to boot up just fine but then computer froze up again. What is going on and how do I fix this?

Sounds like either the hard drive is going or the drive controller is bad. I would immediately back up the drive, then try putting it in another machine (as a slave drive) and see if it works fine.