Windows 7: Troubleshooting unwanted shutdown.

This is an ongoing problem that comes up at startup. I hit the power button on the pc. It turns on, POST,boot up is fine, windows loads. Then just as the logon screen comes up the pc shuts down (well more like turns off!). Most times it doesn't make it that far. Sometimes it turns off at POST. It will even turn off after I log into windows. After it turns off it takes two or three tries to get it to boot up. At first I thought it was the Motherboard but Now I think it is the power supply. Does anyone have any ideas?

I think the PSU may e a good guess. But, if you are overclocking, stop. Go into bios and set optimized defaults and only change the items needed such as boot order and sata controller. Bios will also tell you the voltages it is reading from the PSU. Let us know what they are. But, even so, it does not tell what the voltages are when the system is under strain. If you have another PSU or have somewhere you can borrow one, you could try that. I wouldn't spend money on a new PSU unless I was relatively sure that is the problem. Ram can also be the issue. Here is an excellent tutorial on testing ram. I would let it run overnight or until you see errors. Make sure all your drivers are updated. Try those things and post back.

A tap on the case just solved my problem. I have a short in the wiring to the power button on the case. I'm going to replace the case (its kinda crappy). Any ideas for one in the $50-$100 price range?

Any of the cases on the first 3 pages are pretty good. I don't know about a lot of the cases but the cooler Master HAF series is usually pretty good as well as the Corsair cases. The others I am not familiar with.
Newegg.com - Computer Hardware, Computer Cases, Computer Cases, $75 - $100