My HP pavilion 533w will not power up. The switch seems OK. Is there a fuse that has blown?

Nope. No fuse.

It's either the Power On switch, or the Power Supply.

The main transmission lines coming to your block, and residence, has line transformers. These transformers step down the power coming to your residence. (Ummm, 1440 Volts single phase down to 240 Volts)

There is an adjustment on the transformer. A 'fine tune' adjustment.
120 Volts AC can be anywhere from 108 Volts AC to 127 Volts AC.
No problem. Your appliances, and computer power supply can handle this amount of voltage range.

Same if your country uses 240 Volts.
Range allowed is 208 to 248 Volts.

Guess what happens if the nimrod lineman doesn't check the voltage, before tripping the line transformer on?
Poof!
You may have 164 Volts AC for a 120 Volt system, and brownout your stuff. (Or equivalent for a 240 Volt AC system)

No. The utility company is not responsible. !O_O!

{ I would, or have an experienced electrician, check the voltage coming in to your house. I had to do this on two occasions now.
Yes. 164 Volts for a 120 Volt system. Lineman got two sentences out to the contrary, until I showed him the volt meter readings. Lol!}

Time for you to diagnose.

See if the problem is a $5 ATX Power On switch, or the Power Supply.

A jumper wire is used to bypass the Power On switch.
Does Not directly involve the Power On switch, nor it's wires.

HP Support > Pavilion 533W Desktop PC > Main Support page,

http://h10025.www1.hp.com/ewfrf/wc/product?product=90862&lc=en&cc=us&dlc=en&lang=en&cc=us

Nope. No fuse.

It's either the Power On switch, or the Power Supply.

Diagnose:

1) Unplug the computer from power. Remove all cables.
Looking at the front of the computer is how directions on the computer is given.

The left Side Panel removes. There are two Thumbscrews on the edge of the panel. These are loosened all the way. (Not supposed to come off of the Side Panel)

Pull towards the back of the computer, by the preformed fingergrip in the Side Panel.
Should only pull so far back, then is brought out to the side, and removed.

FOLLOW Anti-Static Precautions

Anti-Static Precautions:
Your body carries Static electricity. Static WILL fry out (Short Circuit), the delicate hardware components inside a computer.
Relieve your body of Static BEFORE reaching inside your computer.

Computer on a table, computer unplugged from power, computer case open;
TOUCH an unpainted surface, of the metal frame of the open computer case.
This action will relieve your body of Static.

IF you leave your computer in the middle of working on it, be SURE to Touch the metal frame again upon your return.

http://h10025.www1.hp.com/ewfrf/wc/document?docname=bph07845&tmp_task=prodinfoCategory&cc=us&dlc=en&lang=en&lc=en&product=90862#N835

The above is an illustration of your motherboard from a Top view, and no hardware components installed.
Scroll down for a photo of the motherboard.

In the illustration look at the top right corner, at Super I/O.
Come down to the vertical placed - ATX Power Connector
Looking at the motherboard photo, you can see it is a White long connector, with 20 socket holes. 10 in each column.

This is the motherboard connector for a 20-pin ATX main power cable, coming from the Power Supply.
This is a general example of a 20-pin ATX main power cable, and it's respective connector on the motherboard,

http://www.playtool.com/pages/psuconnectors/connectors.html#atxmain20

Note the photo to the Right.
Where the wires go in that 20-pin ATX main power cable's connector, is the BACK of the connector.

The wires go down in the socket holes, and end in a metal terminal like this,

http://www.molex.com/molex/products/datasheet.jsp?part=active/0002081202_CRIMP_TERMINALS.xml&channel=Products&Lang=en-US

The wire goes down into the tube shape. The left side is then crimped over the insulation of the wire. It is this side the jumper wire must touch.

The front, or right side in the view, is seen in the middle photo. You can see the tips of all 20 metal connectors.

The preferred jumper wire is a straightened out paperclip, then bent into a U-shape.
The top of the U is wrapped a few times with black plastic electrical tape. This taped area is for your fingers, and thumb to hold onto.

Turning the U-shape upside down it is the 'legs' of the jumper wire that you are going to use.
One 'leg' goes down into a socket hole with the Green wire.
It goes down RIGHT NEXT TO the insulation of the Green wire, and touches the aforementioned metal terminal.

The other 'leg' goes down into a socket hole, with ANY Black wire in it. It also slides past the insulation of the Black wire, slides down into the socket hole next to the Black wire, and touches the metal terminal.

The Green wire is the Soft Power On wire. Abbreviated PS_ON.
ANY Black wire is a Ground wire.

The contact made is no more than 2 seconds.
The Power On switch is a Momentary Contact Switch.

[ The Soft Power On circuit uses 5 Volts DC.
In comparison two D cell flashlight batteries store 3 Volts DC.
Low, low DC voltage, in case you are worried about being shocked.

The high dangerous AC voltage, is contained inside the metal case of the Power Supply.
However when the jumper wire touches there may be a spark.
Warning you in advance. You may wish to wear a glove, to make you feel safer ]

IF, the Power Supply comes on you have a bad Power On switch.
Yes. This IS a common occurrence after a situation like yours.

IF, the Power Supply does Not come on, you have a bad Power Supply. Replace it. Need guidance just post back in a Comment.

The Power On Button is an assembly. The Power On switch is part of it, and is replaceable.
Power On button assembly removed, I use a hair dryer to soften the plastic of the Power On button assembly, and ease the old Power On switch out.
Warmed again to install the new one.

(I'm an old tech)

http://www.directron.com/atxswitch.html

For additional questions please post in a Comment.

Regards,
joecoolvette