The systemax computer has no power at all I

Try Best Buy

A standard ATX power supply. I would buy a PSU with at least 500watts and a 80+ certification. Use a brand such as Chillmaster, OCZ, CIT or Cooler Master. These are all good brands and reliable.

Possible.

No. It can't. Sorry.

if you are not getting any power on the motherboard after you replaced your powersupply then your motherboard has gotten fried and needs to be replaced

Replace the power supply. Also clean all dust out of the unit, do not let fans spin up with any air pressure. Consider replacing all fans, they just fail over time...

Plug a desklamp into the same power point and note whether it flickers at the same time as the reset. If so, you have a power problem. I had a similar problem a while back, discovered the fuse in my household power box was faulty. Replacing the ceramic fuseholder fixed my problem.

This may sound silly, but check the power cord is plugged in hard to the socket at each end. I once made a service call to a computer which wasn't plugged in correctly, and was resetting.

Next most likely cause is the power supply box in the PC itself.

If you would provide more information about your Systemax system... model/serial number I can look the specs up. I can tell you the only way that it could be the hard drive would be if you had a bad power supply, not that your system can't handle the "power" of the hard drive. If you end up getting a new power supply, you could opt for one with a little higher wattage if you'd like. A 160gb hard drive isn't going to pull any more meaningful amount of current than a 40gb. What I would look at would be wither you had the jumpers on the hard drive configured correctly. For info on jumpers, see...

http://en.wikipedia.org/wiki/Jumper_(computing)

Individual drive manufactures and drive models have different method for setting the jumpers and it will also very on what other IDE connected devices you have in your computer (optical drives, hard drives, Zip drives, and etc.).

I recommend that you replace the PSU, Power Supply Unit with a new one, this is a simple task. Remove the plug from Motherboard remove the four screws from the PSU at rear and get a replacement one say a 450 Watt job. Install and ya be good to go.

when you say "the fan makes a noise like its getting ready to spin", which fan...power supply's or cpu? if you answer power supply, then the POWER OK signal is not returning to power supply and it then quickly shuts down to prevent further damage. double check a firm connection to mother board. If you answer CPU, then it becomes far more complicated. Try re-seating the memory chips.