Urgent Sony Vaio VGN-FS315S Help!

[Rendezvous15]

Hi, I own a Sony Vaio VGN-FS315S laptop, with Windows XP Home Edition, and have had it for around 2 years now. A few weeks ago after downloading one of those flash player upgrades, my computer froze and the only option I had was to hold down the 'ON' button to turn it off. When I restarted the laptop, the 'Vaio' logo appeared on the start up screen a lot longer than usual, and then went to a black screen showing the words:

Operating system not found

Nothing would get out of this so I ended up having to restart the laptop several times to find out that my restore system (usually booted by the F9 or 10 key) wasn't working. I was able to enter the BIOS through the F2 key, but little could be changed. The order of boot from the BIOS is set as:

Floppy Disk Drive
Optical Drive
Hard Disk Drive
Network

After trying various repair and boot discs I had found online, and a Windows XP recovery disc from my old desktop (since I don't have any for my laptop), still there was no change. I read up on this error message on the internet and found that other people had recommended using another hard drive. Therefore, I purchased a replacement hard disk drive for my Sony Vaio which is identical to the original. The hard drive is a:

Fujitsu 100GB MHV2100AT (Refurbished)

Replacing the hard drive appeared an easy enough task so I just done it myself. Now with the new drive in the laptop, I get the message:

Invalid system disk
Replace the disk, and then press any key
-

Something I should point out is that the old system recovery disc I previously used had no effect, but this time it did work and reached 100% completion. However, after prompting to restart the system and remove the disk on boot, I returned to the previous error statement of the system disk being invalid.

AHHHHHHHHHH!

Someone please help, I'm getting to the end of my tether with this laptop. I haven't exactly got much money at the moment to take it to a repair shop. Are there any restore discs online that will repair the problem. I'm assuming the new hard drive is fine since I have had more results with that. Although, I would be far more happy to get my old drive with my original files back if possible.

I urgently need my laptop back to complete Uni presentations, so would be extremely grateful to anyone who can help ASAP. Thanks.

[nio_7]

I think that you normally won't face any problem in doing the following.
1) With your brand new empty hard Disk rive inside the laptop, Go in Bios and set the CD-ROM as a first boot device.
2) start your laptop with a Windows XP installation CD, no recovery disks whatsoever!
The setup process would normally start without any problem and you'll be able to setup a fresh install of windows. After that you can install all the drivers for your laptop (download them from sony support web-page)

Now what about your old hard disk...?? It seems that some windows system files were corrupted and the system can no longer start.
But you can do this to save your files located in the old hard disk:
Take the old drive and plug it on another p.c. or on your working laptop ( you will need an appropriate adapter to plug it) as a slave and you'll be able to see all the disk contents.

Thats all i can think of, now..