I have an HP Pavilion Notebook (dv9724tx) running Vista which, whenever i turn it on, switches on the led lights, the fan moves for a few seconds then everything stops, with the screen remaining black. The LED lights stay on, the computer just won't boot into the BIOS. It is not a battery problem and I have tried reseating the RAM. This all occured after the computer suddenly died while booted, then after starting to boot back into windows afterwards, died while loading Vista. It has not displayed anything on the screen since.

Specs:

INTEL CORE 2 DUO T7700 (2.4GHZ/4MB L2 CACHE/800MHZ FSB)
3GB DDR2 RAM
500GB (2X 250GB) 5400RPM HDD
17 INCH WXGA+ (1440 X 900) BRIGHTVIEW DISPLAY
BLU-RAY ROM WITH SUPERMULTI DVD+/-RW DUAL LAYER DRIVE
NVIDIA GEFORCE 8600M GS WITH 512MB DEDICATED VIDEO MEMORY
INTEGRATED 10/100/1000 GIGABIT LAN, INTEGRATED 802.11 A/G/N WLAN, INTEGRATED BLUETOOTH, 56K MODEM
INTEGRATED AUDIO, INTEGRATED WEBCAM, 5-IN-1 DIGITAL MEDIA READER
3X USB 2.0 PORTS, 1X FIREWIRE PORT, 1X HDMI PORT


Thanks in advance to any help received.

Nice machine and configuration.
I would first try removing two memory modules, and attempting a boot with the third.. then if that doesn't work, change modules until you have tried all three.
It is difficult to trouble-shoot a machine like that... but we are seeing a LOT of ealy failures in the large laptops hard drives from 250 GB to 500 GB, so suspect that your hard drive might be bad. A good test would be to try any other known good hard drive from 80 GB on up to 500 GB... can you borrow one, then try to format and install it.
If still dead when you change memory and hard drive, it is time to get it back to HP for warranty work.
Anything you do that is not memory and not a hard drive will void your warranty.

Just to clarify, only 2 memory modules (2gb and 1gb), and I've tested them independently of each other to no avail. Warranty has expired which is why I'm posting here instead of going to HP Support, although I'll contact them anyway since there was a problem with the graphics card at the beginning of the year which they fixed under warranty, so if the current problem's related it can probably be fixed free of charge.

I don't think I have a hard drive that I could test the PC with so I'll try HP first before scouting around for one.

When HP repairs something under warranty. The repaired item's warranty ends when the original warranty would have ended. You do not get an extended warranty of two years or any period longer than the original warranty's end date.
This is also true of any other manufacturer that we know of...
Only a new item that you purchased to upgrade or fix and originally purchased item will have the warranty that comes in the box.

OK, I've called HP Support, a replaced part's warranty only lasts 3 months, so I'll only go to HP as a last resort. It actually booted properly yesterday, although after 5 minutes it died again. If I can get it to boot I'll try to get it into Safe Mode, to see if that helps somehow. Since it looks like a graphics card issue, is it a difficult job to change a graphics card, to see if using another one helps?

Well, it is not really for beginners... but it comes out with a screw driver... then I assume it uses the same drivers as the original.
But I would want to remove or disable the drivers, then install new ones.
We can tell you more how to if you need it.