Where is the battery located in HP Pavilion 533w?

On the motherboard. But you knew that right?

It's a CR2032 battery that is a watch style of battery.

One example,

http://www.radioshack.com/product/index.jsp?productId=3882052

It's about the size of a U.S. nickle.

Getting to the battery, now that's the fun (?) part!

Remove the computer case. (Power unplugged first)

Slides towards the Front, comes off like a hood. On the back of the computer in the TOP middle, and down on the sides are 3 screws to remove. One in the top middle, one on each side.

(The ones on the sides are approximately,... no wait, here let me measure it.........yep, two inches up.
The screws are 2 inches up from the bottom, on each side. I just pulled a Pavilion XG843 out of my shop, which has the same style of computer case)

Look at the computer case frame from the back of the computer.
Notice how it looks as though there are two metal frames?
One of the metal frames makes an L shape. This is on the left side.

See how there's one screw at the top right corner of the L, and one screw down in the right middle of the L?
That's all there is. The other three Philips head screws are for the Power Supply on the right.

Remove the 2 screws in the L shape.

Now looking at the back of the computer once more, go to the Left side. Towards the Front there are two more screws to remove. One 2-1/2 inches down from the top, one 2-1/2 inches up from the bottom.

See the formed 'Bump'? Has a half-moon hole next to it. Place your finger in the hole, raise up under the 'Bump'. The Support Plate swings out from the Front.

(The metal plate the motherboard fastens to, is a Support Plate. Whether it is a separate unit, or an integral part of the sheet metal which compromises the metal framework of the computer case.)

Now look at the inside top right. The CR2032 CMOS battery should be next to the 20-pin ATX main power cable,

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

[PS. Watch the cables, (wires) when you go to put the Support Plate back into position ]

Regards,
joecoolvette