Does it post at all? In other words do you see any text come up on the screen at all? If so, hit the delete key (or whichever key it says to enter setup) to go into setup. Under general or main you should see the IDE devices listed which should show your hard disk. If it says empty for each listing then the BIOS is not seeing the hard drive so it is likely bad.
One of the tabs will allow you to set the boot order. Set it so it boots from CD first.
Get a Linux Live CD and try to boot from that. If you can then it is either the hard drive or software related (OS corrupt, bad drivers, etc.).
If it does boot, try to navigate to your hard disk, if you can see it then it is software related.
Here is a list of live CDs and their primary functions
Note that not all versions of Linux may work with your laptop, and even if it boots you may not get all of the hardware to work due to driver issues etc. That is not important at this point. You just want to see if your laptop will run, period. Even if it gets to the first startup screen with the Linux live CD and locks up or quits there is hope. Try a different Linux version. Knoppix is usually a good one to try as is Ubuntu or Suse.
If you can get it to boot from a Linux live CD then first you want to take out the laptop drive and connect it to another PC using and external drive case for 2.5" drives and see if you can get the data off.
If you can get the data off then restore the system using the rescue disks that came with the laptop. If the drive can't be seen or read from then you need to get another laptop drive and re-install using the rescue disks that came with the laptop.
If it will not boot at all with the Linux live CD then it is a hardware problem such as bad motherboard, processor, etc.
Good luck
When you first turn on the laptop do you get the HP Welcome Screen?
Yes . Press whatever key it needs to enter setup. Go to the final screen using the arrow keys and set it at the default values. Then try to re-boot to windows
NO. Motherboard, CPU or Hard Drive has died.
good call on the memory.