A brief primer if I may;
Electrolytic Capacitors are used on the motherboard, and in the Power Supply, in the Compaq Presario 1420nx Desktop PC;
HP Support > Compaq Presario SR1420nx Desktop PC > Main Support page,http://h10025.www1.hp.com/ewfrf/wc/product?cc=us&dlc=en&lc=en&product=457247&
Product information > Product specifications > Motherboard Specifications, P4GV-LA (Guppy),http://h10025.www1.hp.com/ewfrf/wc/document?docname=c00363478&tmp_task=prodinfoCategory&cc=us&dlc=en&lc=en&product=457247
Scroll down to the motherboard Photo.http://capacitorlab.com/visible-failures/index.htm
General example of a Switched-Mode Power Supply, (SMPS), case open,http://en.wikipedia.org/wiki/File:ATX_power_supply_interior-1000px_transparent.png
Personal computers have used this style of Power Supply, for quite some time.
The one in your computer is just an ATX style, and is the type used in MANY personal computers. (Not Small Form Factor, however)
THIS IS NOT an invite to open your Power Supply, and attempt repair.
The capacitors used are Radial Aluminum Electrolytic Capacitors.
The blue ones marked by the letter B, are high voltage. MAY have Up To 100 Volts stored.
Your fingers touch the terminals on the bottom, of one of these babies; the shock can be BAD to FATAL!
Your fingers touch a circuit, that one or more capacitors are in, and the shock can be BAD to FATAL.
There is a Proper method to discharge them, BEFORE working on the unit. Am not going to post.
(Does NOT involve a metal screwdriver, or other metallic object! Good way to have hot molten metal spray in your face!)
The method of repair is to REPLACE.
Inside an Electrolytic Capacitor is Electrolytic Paste.
This paste breaks down over time. Chemical composition breaks down.
Computer engineers know this, and use capacitors that are rated at twice, or 200 percent; of what is needed.
This way when the capacitor breaks down to 50 percent good, it is still 100 percent good for the application it is used in.
Unplug an electronic unit from power, and the chemical composition of the paste breaks down more rapidly. All electronic units need a trickle charge when being stored. (Pawn shops know this)
FOLLOW Anti-Static Precautions after unplugging your computer from power, BEFORE reaching inside your computer. (Don't know, and would like to? Post in a Comment)
Check the 'caps' on the motherboard.
Note* Sometimes the Electrolytic Paste dries up, and shows NO outward visual signs of failure.
'Caps' on motherboard found to be good, suspect the Power Supply.
I can guide you in testing it, or you may have a KNOWN to be good, Compatible power supply, from a working computer that you can borrow for a test unit.