Jono,
when the computer starts up and the bios posts you should be able to see the hard disk and its SMART status...it may say ok or something like degraded...
If you just see a motherboard splash screen, hit tab to show the detail,
once it gets by that screen and you have to choose the OS, as soon as you choose XP and hit enter, immediately start pressing F8 and tell me if you get another boot menu,
if you do, try to go into safe mode or command prompt!
Have you got a setup cd? You can also try running the recovery console -
How to use the Recovery Console
You can enable and disable services, format drives, read and write data on a local drive (including drives that are formatted to use the NTFS file system), and perform many other administrative tasks. The Recovery Console is very useful if you have to repair your computer by copying a file from a disk or CD-ROM to your hard disk, or if you have to reconfigure a service that is preventing your computer from starting correctly.
If you cannot start your Windows, you can run the Recovery Console from the Windows XP startup disks or the Windows XP CD-ROM.
After Windows XP is installed on your computer, to start the computer and use the Recovery Console. The Windows XP startup disks or the Windows XP CD-ROM are required.
For more information about how to create Startup disks for Windows XP (they are not included with Windows XP), click the following article number to view the article in the Microsoft Knowledge Base:
310994 Obtaining Windows XP Setup boot disks
Note To start the computer from the Windows XP CD-ROM, you must configure the basic input/output system (BIOS) of the computer to start from your CD-ROM.
To run the Recovery Console from the Windows XP startup disks or the Windows XP CD-ROM, follow these steps:
1. Insert the Windows XP startup disk into the floppy disk drive, or insert the Windows XP CD-ROM into the CD drive, and then restart the computer.
Click to select any options that are required to start the computer from the CD drive if you are prompted.
2. When the "Welcome to Setup" screen appears, press R to start the Recovery Console.
3. If you have a dual-boot or multiple-boot computer, select the installation that you must access from the Recovery Console.
4. When you are prompted, type the Administrator password. If the administrator password is blank, just press ENTER.
5. At the command prompt, type the appropriate commands to diagnose and repair your Windows XP installation.
For a list of commands that are available in Recovery Console, type recovery console commands or help at the command prompt, and then press ENTER.
For information about a specific command, type help commandname at the command prompt, and then press ENTER.
6. To exit the Recovery Console and restart the computer, type exit at the command prompt, and then press ENTER.
It doesn't sound like its totally lost since you at least get an OS boot menu...