Win XP will not start

We first need to make sure XP is detecting the hard drive, are you getting to that point in the setup, where you see the partition screen? If so, you can try setting your SATA options in BIOS to 'Compatible', and try wiping your CD off before reloading.
If you are seeing your hard drive from BIOS, but not from the XP install you might need to F6 during the initial setup screen to load your SATA drivers.

You can also run diagnostics on the hard drive, which you download from the hard drive manufacturer, Seagate, Western Digital, etc... all have their own diagnostic utilities available for download on their respective websites (usually a CD you burn and boot off of)

1).

Repair Install
If XP is corrupted to the point where none of the previous solutions get it to boot,
you can do a Repair Install that might work as well as keep the current settings.
Make sure you have your valid WindowsXP key.
The whole process takes about half an hour depending on your computer
If you are being prompted for the administrator's password, you need to choose the 2nd repair option, not the first.
1.Insert and boot from your WindowsXP CD
2.At the second R=Repair option, press the R key
3.This will start the repair
4.Press F8 for I Agree at the Licensing Agreement
5.Press R when the directory where WindowsXP is installed is shown. Typically this is C:\WINDOWS
6.It will then check the C: drive and start copying files
7.It will automatically reboot when needed. Keep the CD in the drive.
8.You will then see the graphic part of the repair that is like during a normal install of XP (Collecting Information, Dynamic Update, Preparing Installation, Installing Windows, Finalizing Installation)
9.When prompted, click on the Next button
10.When prompted, enter your XP key
11.Normally you will want to keep the same Workgroup or Domain name
12.The computer will reboot
13.Then you will have the same screens as a normal XP Install
14.Activate if you want (usually a good idea)
15.Register if you want (but not necessary)
16.Finish
17.At this point you should be able to log in with any existing accounts.
.............................................
2).

NTLDR or NTDETECT.COM Not Found
If you get an error that NTLDR is not found during bootup,
If you have FAT32 partitions, it is much simpler than with NTFS.
Just boot with a Win98 floppy and copy the NTLDR or NTDETECT.COM files
from the i386 directory to the root of the C:\ drive.
For NTFS:
1.Insert and boot from your WindowsXP CD.
2.At the first R=Repair option, press the R key
3.Press the number that corresponds to the correct location for the installation of Windows you want to repair.
Typically this will be #1
4.Enter in the administrator password when requested
5.Enter in the following commands (X: is replaced by the actual drive letter that is assigned to the CD ROM drive.
COPY X:\i386\NTLDR C\:
COPY X:\i386\NTDETECT.COM C:\
6.Take out the CD ROM and type exit

Power issues at your house? sags and surges? Lightning storms?

Is your power supply going bad? Maybe it doesn?t have enough juice to keep your drive spinning at right speed.

How do your cables look?

Try a different SATA port on the motherboard?

And check the HDD for correct Function. You can get the Testing Utility from the HDD Makers Web Site or you can use this URL to navigate to it.

http://www.tacktech.com/display.cfm?ttid=287

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Insert the Windows XP bootable CD into the computer.
When prompted to press any key to boot from the CD, press any key.
Once in the Windows XP setup menu press the "R" key to repair Windows.
Log into your Windows installation by pressing the "1" key and pressing enter.
You will then be prompted for your administrator password, enter that password.
Copy the below two files to the root directory of the primary hard disk. In the below example we are copying these files from the CD-ROM drive letter, which in this case is "e." This letter may be different on your computer.

copy e:\i386\ntldr c:\
copy e:\i386\ntdetect.com c:\

Once both of these files have been successfully copied, remove the CD from the computer and reboot.