[newbe]

I am having problems with my desktop PC.. I have had a new hard drive installed around 6 months to a year ago (not exactly sure when) and never had problems with it in the beginning. Now whenever i switch on the PC it takes longer than usual at...

I am having problems with my desktop PC..
I have had a new hard drive installed around 6 months to a year ago (not exactly sure when) and never had problems with it in the beginning.
Now whenever i switch on the PC it takes longer than usual at the first black start up screen, then it takes me to the next black screen which reads:
DISK BOOT FAILURE, INSERT SYSTEM DISK AND PRESS ENTER.
I decided to open up the PC casing and have a look inside, i unplugged the HDD and then replugged it in and switched the PC on. To my surprise, the PC was now working again and i thought i had fixed it. Although..
When on the computer, i randomly get a blue screen that comes up which reads:

A problem has been detected and windows has been shut down to prevent damage to your computer.
A process or thread crucial to system operation has unexpectedly exited or been terminated.
If this is the first time you've seen this stop error screen, restart your computer. If this screen appears again, follow these steps:
Check to make sure any new hardware or software is properly installed. If this is a new installation, ask your hardware of software manufacturer for any windows updates you might need.
If problems continue, disable or remove any newly installed hardware or software. Disable BIOS memory options such as caching or shadowing. If you need to use Safe mode to remove or disable components, restart your computer. Press F8 to select Advanced Startup Options, and then select Safe mode.
Technical Information (a whole bunch of letters and numbers)
Beginning dump of physical memory.

So i restart and put in my Windows XP disc and press enter, it starts up with the blue screen for windows setup and loads my files. But then the next screen i see reads:

Setup did not find any hard disk drives installed on your computer.
Make sure any hard disk drives are powered on and properly connected to your computer, and that any disk-related hardware configuration is correct. This may involve running a manufacturer-supplied diagnostic or setup program.
Setup cannot continue. To quit setup, press F3.

So i press F3 which restarts the computer which then leads me once again to the black screen with the error message.

When my PC starts up i hit delete to enter SETUP. I enter Advanced BIOS Features and set my First Boot Device as HDD-0 which is Boot from First HDD. There is also the option to choose HDD-1,2 or 3, i've tried all of these but nothing solves the problem. My second boot device is CDROM, third is Floppy and so on.

So please if anybody could help me on this issue i would really appreciate it! I have a laptop but my sister desperately needs to use this PC to do school work, and also because she has work saved on it. I also have a whole heap of stuff saved on the HDD and was hoping i dont lose it all.

Thanks in advance
Francesco