computer not functioning properly

Hi,
I am totally out of ideas to try to get my comp to work the way it once did. My first issue is the hard drive. The hard drive shows up in bios sometimes and doesn't other times and it, or something in the computer is making a low beeping noise that is constant. When it makes the beeping noise and I go into the bios the hard drive is not showing up. When it has no noise, it boots up fine but then freezes or shuts down within 30 minutes or so. Also when i leave the computer on for a while it goes to the screen saver and then it goes into what seems to be in sleep mode, but it is really shut off and I have to hold the power button in for several seconds before the system will even power on. I know its not the cmos cuz i have already replaced that. I even did a check with a 3rd party software to check my hard drive, and motherboard and everything appeared to be normal. I am thouroughly at wits end with this one and desperately need help. I am a gamer and haven't been able to play my world of warcraft game in months...please someone help me!!!!