HP Pavilion Slimline won't start up

i would suggest you to do a software update.
it may also be because of the service provider

The beeps are a code that tells you what is wrong and they vary by BIOS manufacturer. The reference is here:
http://support.hp.com/us-en/product/hp-pavilion-slimline-s3600-desktop-pc-series/3999463/model/4006368/document/bph07107

No Power on an HP Pavilion PC Desktop b> Desktop computers are simultaneously more complex and easier than laptops when it comes to troubleshooting. There are more physical components on a desktop, so locating a specific troublesome part can be tricky, but working inside of a desktop computer is straightforward compared to opening and fiddling with a laptop. Power problems are always hardware problems. There are a few simple, preliminary steps to troubleshooting, but power problems often require opening your computer and having a look inside.

The Basics
Unplug everything from your PC except for the power cord. Try turning on the system and look for lights on the front, then on the back. When the power supply is on (receiving power), the small light beneath the plug turns green. If this light doesn't come on, the problem is either connected to the cord or the power supply. Try swapping power cords with the monitor cord to isolate the cord as the source of the problem. Look closely at the voltage selector and make sure it's set appropriately for your country -- 110 in North America and 220 for most other regions. b> The Socket b> Unplug the power cord and switch the voltage selector to the opposite setting. Wait five seconds, then flip it back. This ensures that the switch is engaged. Plug in the power cord again and try turning on the PC. Check the outlet to make sure it's working, and plug your computer direction into the wall socket (remove it from any surge protectors or power bars). Check the power button. If it sticks when you push it, it may not be making contact to engage the system power and will need to be replaced. If the button is going in and out freely, you've done about everything you can do from the outside of the computer. It's time to go inside. b> Open it Up b> Unplug the power cord and hold down the power button for five seconds. This clears all electrical charges from the circuitry and makes it safe to open the computer. Remove the side panel from your system. This process varies a bit from model to model -- some have one or two captive screws on the back of the system, some have an easy release button. Check your manual. b> Power Button b> Locate the power button from the inside and trace the cables to where they plug into the motherboard. Lift up on the connector and then push it firmly back down. Plug the PC back in to its power source and try to power it on. Unplug the computer immediately if the system doesn't power up, and continue troubleshooting. b> Hardware Components b> Disconnect each of the devices in your computer one at a time to locate a faulty piece of hardware. Power cables run from the power supply connect to your optical drive, hard drive, floppy drive (if one exists) and into several places on the system board. This step takes some trial and error detective work and a few minutes: unplug one device, reconnect the power, and try turning on the system. Then disconnect the AC power, plug the device back in and try the next one. If the system powers up at any stage, the device you are unhooking is shorting out the power and needs to be replaced.
http://www.youtube.com/watch?v=HJvYGtbwmh8 To Replace HP Pavilion Desktop Power Supply http://www.youtube.com/watch?v=VcEFUpZSmI4 Hp Pavilion 750n desktop computer basics on this computer b> http://www.youtube.com/watch?v=D8Y2PPMiiB0 HP Pavilion Slimline PSU Upgrade Guide This guide compares the Sparkle SPI270LE - 270 watt PSU against the Shuttle PC50 -300 watt as possible power supply upgrade for a HP Slimline. The s3600t Slimline I used for this video has a Intel Hope this helps.

If you saying "doesn't always boot", most common would be motherboard...
If your motherboard still beeps, then probably some other components has the problem like RAM, GPU card etc., depends on your motherboard.

Hope that give an idea...

Hi there,
please check if you can see any light or LED display when you press the power button.
If none. Check if the power cable is tight on the back of the computer and check if it's plugged into a power outlet.
You can also try plugging it into a different power outlet just to make sure it's not the Power outlet that has a problem.
You can also try to switch the power cables.
Hope this works, if it doesn't you will have to tear down the parts of the computer to check where the fault is coming from.

Try this first.
Unplug the power cord from the wall outlet for about ten minutes then plug it back in and see if your PC powers up.
If still no power, the power supply is shot and will need to be replaced, it costs about $40 for the part.
I hope this helped.
Good luck and have a pleasent day.

Does it have power? Have you tried hitting the F8 KEY repeatedly after the initial windows screen starts? Do any of the pc's lights come on (hard drive usage,lan,cap lock,num lock, etc)? If you have absolutely no power try another outlet, if your connected to a surge protector see if the surge protector reset has tripped (found on medium to expensive protectors - if you hve a inexpensive one you probably wont have a a reset on it) try to plug something else into the outlet like a lamp, if you have no power check your household circuit protector and reset the fuse, but of course be leery. If something tripped the circuit something isnt right, indicating a power surge. Storms in your area or construction nearby?

Does the computer turn ON? (do you hear beeps?, do you see lights on the computer NOT THE MONITOR?, do you hear the fans?, etc...) it could be the monitor, try an other one, it could be the video card, swap it with a known good one ,it could be the VGA cable change it, etc... if the computer is all dead try an other electric outlet.

Good luck!

check if your power used to connect form your CPU to the outlet is not defective...use a multitester to test the said cable,,or simply use another power cord and conenct it from your cpu to the outlet...if still the same you have to purchase a new power supply since there is no power and your curretn power supply is already defective.

taking out the battery resets the settings on the bios for the motherboard.

worked for me on the few computers that wont turn on.

If it starts up, you may need to enter dates etc onto the bios.