buy a whole goddamn transformer
A computer Power Supply generally has a transformer in it, depending on the design.
So just buying a transformer would not work out well for you since the voltage would still swing from +12V to -12V. This would cause your computer to release all of its magic smoke.
You also need a rectifier to convert the AC signal into a DC signal as shown below
![](http://upload.wikimedia.org/wikipedia/commons/thumb/5/5c/Fullwave.rectifier.en.svg/760px-Fullwave.rectifier.en.svg.png)
But notice how the signal still has valleys in it. If you hooked this up to your computer, it would still explode, or at least reboot 120 times a second.
Then you add capacitors, inductors and other items to "smooth" out the signal.
However, computer power-supplies are generally Switching Power supplies, which allows them to be up to 90% efficient. If they instead were a linear power supply as a was showing above you would struggle to get 30% efficiency.
As for a Computer power-supply that is ~650W and ~$90 I would recommend either:
http://www.newegg.com/Product/Product.aspx?Item=N82E16817703036or as Supertoaster suggested:
http://www.newegg.com/Product/Product.aspx?Item=N82E16817139005