Won't power up

That power supply that you bought is way too big. See this unit http://www.circotech.com/replacement-hp-pavilion-micro-atx-ps3-compatible-replacement-power-supply.html

This is almost always the power supply. How exactly did you "check the power supply"? You checked all 12 or so voltage outs under load?

If replacing the power supply doesn't work, replace the motherboard.

The specs say the PSU is ATX compatible. Whether that is for the connector only or for the whole power supply is something you will have to determine.

If you have an ATX power supply handy (like in an old computer) give it a try. This one is only 300 Watts so it won't likely be expensive to replace.

Have a look at: http://www.techspot.com/community/topics/dell-dimension-1100-shuts-down-after-20-seconds.135987/

This is a very common symptom of a failure in the power supply board. The failure is caused by parts called capacitors. Here is a link to information on what needs to be replace on the power supply board to repair it. It is an easy repair, takes about 20 minutes.

http://www.ccl-la.com/catalog/product_info.php?cPath=95_97&products_id=216

Take the power supply to a retailer or computer shop in your local area to be tested. Pretty standard from what I have found out.
Bestec ATX 300 12Z Rev DDR Replacement Power Supply HP 5188 2625 Have a great day.

If you need further help, I’m available over the phone at https://www.6ya.com/expert/william_02fc2766c1e13b73

If your ATX switch mode power supply is faulty then I suggest you replace it with a new one that is 350 watt or higher wattage.
Fixing faulty ATX power supplies is not a DIY project, there are dangerous voltages in these power supplies and can cause death or serious injuries.

Most computer testing facilities will test your current power supply for free. A replacement is very inexpensive to replace. Bestbuy, MicroCenter, small computer shops and buy replacemnent from Amazon.com, newegg.com.. Just make sure you have the correct information for your replacement. The model number on the power supply would be your best bet in most cases. Good luck and have a great day. I buy power supplies as low as $15 dollars Generic

If you need further help, I’m available over the phone at https://www.6ya.com/expert/william_02fc2766c1e13b73

Although ATX power supplies conform to various specification guidelines, there is not, unfortunately, a standard ATX power supply. To find a suitable replacement power supply for your machine, we would recommend that you have the following information to hand when making your purchase:

- You will require a PSU with a 24-pin ATX connector, SATA power connectors, and a 4-pin 12V motherboard connector
- Make a note of the wattage of your existing PSU
- Make a note of the external dimensions of your existing power supply
- Be aware that your PSU has a power connector for speakers. Most PSUs do not have this, so you may need to purchase a separate power supply for your speakers.
So it is a slight variation to a standard ATX power supply. If you are not using the speaker power off the power supply, then any standard ATX power supply will work for you

The old AT power supply that these old computers uses has two six pin socket connectors and these are different to the ATX connector.
You cannot buy these power supply now, most likely you might find one/computer in a junk shop.
If you have electronic knowledge you might be able to cut off the connectors from the old power supply and connect it to the ATX power supply and modify the ATX power supply so you can switch on the power to the old computer. The old AT power supply uses a mains power switch whereas the ATX uses low voltage to turn on the power supply.
Good luck & have fun.

yo dawg just do like buy a new connector and stuff you know what i mean dawg