Hopefully your important files are backed-up. If not, maybe find a way to backup your files first, perhaps by using a bootable linux CD (such a Knoppix - search on internet for more info ) which may allow you to have access to the files on your hard-drive and give you the ability to copy them over to another source, such as an external USB hard-drive or other storage media.
But anyways, it sounds like perhaps the boot file or some other important Windows files somehow got deleted or corrupted.
Chances are that not all is lost though.
You can probably try to do a "Repair Install" which can hopefully be able to fix and repair and missing or corrupt Windows files, as well as it should still leave your files and settings intact.
You will need to use your Windows Installation CD that came with your computer to try this Repair Install option. If wanting to try the Repair Installation, place the Windows Installation CD into your computer and then reboot / turn-on the computer.
This answer will be divided into two-posts here, as it's too long to have all in 1 post appearantly (it exceeds the allowable max 2000 character count)
1. Boot the computer using the XP CD. You may need to change the boot order in the system BIOS so the CD boots before the hard drive. Check your system documentation for steps to access the BIOS and change the boot order.
2. When you see the "Welcome To Setup" screen, you will see the options below
This portion of the Setup program prepares Microsoft
Windows XP to run on your computer:
To setup Windows XP now, press ENTER.
To repair a Windows XP installation using Recovery Console, press R.
To quit Setup without installing Windows XP, press F3.
----> continued in my next post here: