As mentioned above, look in BIOS set up to make sure that the hard drive is BOTH designated as BOOTABLE and is in the BOOT ORDER as the first or second device behind the DVD drive.
If the drive does not show up in the BIOS at all then try replacing the SATA power and signal cables to the hard drive. If that does not help then it is the hard drive(s) or the motherboard's SATA controller.
Have you checked all the HD connections. Pull them in and out a few times before pressing them home firmly again, to clear any oxide.
Most likely it is HD failure. Just because it is only 4 months old does not preclude that. Faults on any device (not just computers) were always OK until they suddenly were not, so it doesn't tell us much.
Most likely a motherboard/BIOS issue then. If the HD doesn't even show in BIOS it is not a Windows problem so software will not fix it.
› USB mouse is not detected in XP? any ideas?
Why did you have to replace the HDD on a 1 yr old laptop? It's possible the new HDD has failed or maybe it just came loose? Did you change the boot order settings in the BIOS? Your laptop is trying to boot off a network drive, that's what the PXE error is all about. No network drive is being found, therefore "Operating System not found".
You might still be covered under warranty - even if a little over one year old.. Depending on where you're located warranty terms etc. both vary and also have some built-in flexibility - not the least in UK and Europe.
Might be worth leaning on the vendor/manufacture at least to see what might be possible?
Are you able to remove the hard drive at least; and then perhaps connect it via usb adapter to another computer? If so then you can both verify the drive is basically OK; and even copy any personal files to external storage (DVD etc.) as a safeguard.
Can you access bios settings? If so then try reseting them to defaults at least?
Poster says another HD was tried - see response #2.
Can you confirm that the HD is not visible in BIOS - that is how I read your post?
Can you also confirm that you did not flash the BIOS (update) just before this issue came along, or make any changes there?