Make sure you don't have a floppy in the drive when booting.
Perhaps this information will be of assistance to you:
Possible cause is a damaged boot sector or a missing or corrupt ntldr or
ntdetect.com files. Use the COPY command to replace damaged files from the
Win XP CD.
Boot from the Win XP CD and enter the Recovery Console. At the Command
Prompt type the following ( "X" is your CD-Rom drive letter). Allow the
files to overwrite the old files
COPY X:\i386\NTLDR C:
COPY X:\i386\NTDETECT.COM C:
A damaged Boot Sector can be repaired at the command prompt by typing
FIXBOOT and press Enter. Then answer "Y".
http://support.microsoft.com/default.aspx?scid=kb;en-us;307654
How to use the Recovery Console
http://support.microsoft.com/default.aspx?scid=kb;EN-US;314057
"NTLDR Is Missing" Error Message When You Upgrade or Install Windows XP
Over Windows 95, Windows 98, or Windows Millennium Edition