I also think it has something to do with the SMPS.
MOST computer failure can be attributed to Power Supply failure.
MOST computers out there are dirty inside, as well as the Power Supply.
Also, your diagnosis of the green light going out indicates PSU failure.
(Power Supply Unit)
The two cooling components for an SMPS are it's Fan, and any Heatsink's used inside.
Generally a metal plate that has tall, thin fins protruding from it.
The metal plate absorbs heat from whatever object it is placed against, and the fins absorb heat from the metal plate.
The fins then radiate the heat away.
If a cooling fan is used in conjunction with the Heatsink, the air movement helps to dissipate the heat, that has been absorbed by the fins.
Internal view of an average SMPS,http://en.wikipedia.org/wiki/Switched-mode_power_supply
Upper right photo. (You can enlarge the photo by Left-clicking on it. You can enlarge it twice)
Once the cooling components of the SMPS are clogged with 'Gunk', the cooling capacity drops tremendously.
Heat = Wasted Energy
The SMPS tries to keep up with the call for power, but eventually cannot, and hardware components inside the SMPS fail.
NEVER open a SMPS, and try to repair it, unless you are a qualified tech. A qualified tech, knows the proper procedure to drain the power from the capacitors, before working on the unit.
(Frankly it's not worth the effort. The procedure is to just replace the SMPS)
Inside the SMPS are Electrolytic Capacitors.
They are used as Filters.
They filter the incoming AC electricity, (Input Stage), and the outgoing DC electricity made by the SMPS. (Output Stage)
An Electrolytic Capacitor slowly builds up a charge, and then releases it all at once.
You may compare it to a large swimming pool being filled up by a garden hose, then one wall of the pool is taken down all at once.
Electrolytic Capacitors hold a charge for Weeks, Months, or even over a year, in some instances, after being removed from power.
(In this instance it would be unplugging the PSU from power)
IF, your finger/s touch the terminals on the bottom of a Radial Aluminum Electrolytic Capacitor, the charge could be released to You!
IF, your fingers complete a circuit, that one or more of those capacitors are in, the charge could be released to You!
Shock can be Bad to Fatal!
(See those two large Electrolytic Capacitors, in the SMPS link photo I provided?
Upper left corner of the SMPS photo. They have blue plastic sleeves around them. They are indicated by the letter B.
These are Electrolytic Capacitors used in the Input Stage. If they are good, they have enough power stored to put X's in your eyes)
Ready to test this puppy (SMPS), or replace it? Just post in a Comment. (Believe upper right of your page)
(The SMPS in your computer puts out three main Voltages.
A) 3.3 Volts (DC)
B) 5 Volts (DC)
C) 12 Volts (DC)
Two C cell flashlight batteries produce 3 Volts (DC)
The test is done with a multimeter, or a power supply tester.
One example of a power supply tester,http://www.tigerdirect.com/applications/SearchTools/item-details.asp?EdpNo=5250576&CatId=5471
Buy a new Harddrive for it? Why?
If you do, be prepared to buy another genuine copy of Windows.
Information on the Compaq Presario SR1530F desktop computer;
HP Support>Compaq SR5301F desktop computer>Product Specifications main menu,http://h10025.www1.hp.com/ewfrf/wc/documentSubCategory?tmp_rule=51284&tmp_task=prodinfoCategory&lc=en&dlc=en&cc=us&lang=en&product=3644734