As a hedge against being unable to start XP, you can create a boot disk specifically for Windows XP that may allow you to boot the operating system. This can occur when the active partition boot record or other required files become corrupted.
Using Windows XP, place a blank floppy in drive A and format the disk. Navigate to the root folder of the system (in most cases this will be C:\) and copy the following files to the floppy disk.
If you can't see the files, click [Tools] and [Folder Options...] in the Menu Bar of Windows Explorer. Click the [View] tab and click the radio button next to [Show Hidden Files and Folders]. Remove the check mark next to [Hide Protected Operating System Files (Recommended)] then click [Apply] and [OK]. The files should now be visible in Windows Explorer. If the files still cannot be copied, right click each file, click [Properties] and remove the checkmark from the Read-Only attributes. Reset whatever attributes were unchecked after the copy procedure is completed.
In addition to the files listed above, check the same directory for bootsect.dos and ntbootdd.sys files. If they are present on your system, copy them to the same floppy diskette.
The disk is used as any other boot disk, inserting it into the floppy drive (A:\) and initiating a cold boot. Make sure the BIOS is set so that the A:\ drive is first in the boot sequence.