My computer turn on and off contantly just stay 5 second on and then turn off same thing

Is the computer dirty inside, as well as the power supply?

Inside your computer is a Heatsink, sitting on top of the Processor.

The Heatsink is essentially a block of metal, that has tall thin fins protruding from it.

Heat is transferred to the block, and is radiated out of the thin fins.
A fan pushes air through the fins, and the radiated heat is carried away.

Example of an average Heatsink/Fan combo,

http://www.tigerdirect.com/applications/SearchTools/item-details.asp?EdpNo=2943299&CatId=493

If the fan that pushes air over the Heatsink, is clogged with dust, and dirt, as well as the fins of the Heatsink, the cooling capacity for the Processor will drop tremendously.

When a Processor overheats it turns off. (BIOS turns it off)
The Processor has a thermal limit, and will eventually burn up if it goes past this limit.
This Fail Safe feature that is built-in, keeps the Processor from overheating, by turning it off.

Easy solution for this is to unplug the computer from power, open the computer case, and use a can of compressed air for computers to blow the dust, and dirt out.

NOTE*
BEFORE you reach inside your computer for any reason, you need to relieve your body of Static electricity. (Not shouting)

Static will fry out, (Short Circuit), the computer's hardware components, inside the computer.

All you have to do, is touch the metal frame of the open computer case.

IF, you get up for any reason in the middle of working on your computer, walk away, then return, TOUCH the metal frame of the open computer case again.

Computer unplugged from power, you are safe.
Touch the metal frame, your computer is safe.

Break the lock tab off of the top of the can of air, insert the plastic straw.

Start at the top left inside corner of the computer, as you are facing the opening, and go across to the right.
Drop down a row, of what you think the right distance might be, shoot air across again.
Keep going until you reach the bottom of the computer case.

Pay special attention, to cleaning any dust out of in-between those Heatsink fins. This area needs to be Clean.
Refrain from using a steady blast of air on fans.
Use Short Bursts.
(Squeeze the trigger in all the way, then let go quickly)

The fans are only designed to spin so fast, and spinning them faster than they were designed for, can lead to premature failure of the fan's bearings.

Also use the straw to clean the Power Supply out through it's ventilation holes. Shoot air from the inside of the computer, on this side of the Power Supply.

Then come around to the back of the computer, and shoot air on the fan blades, center hub, and surrounding cage of the fan.
Finally come around to the inside of the computer again, and shoot air along the bottom.

The second thing it could be is a failing power supply.
To test if this is the problem you can buy a cheap power supply tester, or use a known, compatible, good power supply for a test.

If you would like more information on this, please post in a comment.

The last thing that could be the problem is the motherboard, and specifically Electrolytic Capacitors that have failed, or are failing.

This link gives you information of identifying Electrolytic Capacitors, what they look like on an average motherboard, and the visual signs of capacitor failure,

http://www.capacitorlab.com/visible-failures/index.htm

This link is to the Service Manual for the Dimension 2350 Desktop PC, from Dell Support,

http://support.dell.com/support/edocs/systems/dim2350/

THIS SHOULD HAPPEND WITH RAM PROBLEM REMOVE THE RAM ONCE AND CHECKOUT THE SLOTS ONCE CLEAN THE RAM WITH COTTON CLOTH THEN INSERT PROPERLY IF IT IS NOT WORKING TRY ON OTHER SLOT ON THIS ALSO IT IS NOT WORKING THEN YOU SHOUL CHANGE YOUR RAM