Bought a computer and it's broken, wondering what could be wrong?

How much experience do you have troubleshooting a system? If you don't have any, I would simply turn off the system and unplug the power supply. Remove the motherboard battery for one minute, and reinstall. This will reset the bios and might get you a post screen or some beep codes if your board has a built in speaker. If you don't care about the money you've spent, then you can learn how to troubleshoot by removing or disconnecting components until it posts if you're lucky. I would normally suspect the power supply or motherboard, but it could be something else, like a bad hardrive. Start by disonnecting the hardrive connectors, and try posting. Also check youtube or google for troubleshooting videos. They're out there; plenty to choose from.

It's most likely something wrong with the motherboard. Does it have on-board graphics or a discrete graphics card?

Sounds like Ram beeps to me, count the beeps and google 'HP Pavilion error beeps' see what the net can bring up,
other than that its reading part no's on the components like the mobo to id them, and then go from there
Moto

take it back and see what the "dodgy" computer store is willing to do for you.

Probably call the 'sold as seen mate' dept,
only thing Op can do is try to rescue it, or learn from the experience
Moto

Alright, I have a friend who's a bit more knowlegable coming over and hopefully he can help with this, as I have almost no clue about any of this..

Cool, let us know how it goes man  
Moto

Will do.

Update guys, I've managed to get it working, I used my old tower's graphics card and ram and it now boots fine, so hopefully now I'll look for a good graphics card and a good ram drive that fits.
Thanks for the help though guys.
FJTheGamer.

Glad you got it resolved man, try the card in another machine to test, its possible you can reflow it and it will be fine
Moto