Can you borrow an XP CD to use the Recovery Console tools to try to repair your machine? If so, attempt to recover the existing O/S through the Recovery Console.
Use this to get yourself into the Recovery Console;
Description of the Windows XP Recovery Console for advanced users
When you get to the command prompt, firstly type in
chkdsk c: /f and hit ENTER. If this fails use
chkdsk c: /r and hit ENTER. If errors are found and corrected, attempt a normal boot.
If it doesn't boot, use the CD to get back to the command prompt as above. Then type in
bootcfg /rebuild.
A discussion about the Bootcfg command and its uses Attempt a normal boot.
If it doesn't boot, use the CD to get back to the command prompt as above. Next replace the potentially damaged ntldr and ntdetect.com files. You can copy fresh files from the XP CD using the COPY command.
At the Command Prompt type the following (where "X" is your CD-Rom drive letter) allowing the files to overwrite the old files
COPY X:\i386\NTLDR C:
COPY X:\i386\NTDETECT.COM C:
Attempt to boot normally. If it doesn't boot, again start with the CD to get to the command prompt. Type in
fixboot c: and hit ENTER. When it finishes, attempt a normal boot.
If it doesn't boot, again start with the CD to get to the command prompt. Type in
fixmbr c: and hit ENTER. When it finishes, attempt a normal boot.
If all this fails to recover the existing O/S, then you may have to purchase an XP CD and reinstall XP on that machine.