|Several reasons :|
The Default value in the [Boot Loader] section of the Boot.ini file is missing or invalid.
Solution : Replace boot.ini file with recovery console
Windows XP is not installed in the location specified in the Boot.ini file.
Solution : Also, boot.ini may be bad
The Ntoskrnl.exe file is missing or damaged.
Replace ntoskrnl.exe with recovery console
The partition path in the Boot.ini file is not set correctly.
Solution : Replace boot.ini
General hardware failure.
If replacing of boot.ini doesn`t help, go here and read about Recovery console
read the example and replace the lines
did you install xp in soem other partition ( not the first listed ) ???
our patience is big