Boot from a windows installation CD, chose repair on the first screen using recovery console.
After choosing the windows installation that is corrupt type bootcfg /rebuild, type Y for Add installation to boot list? (Yes/No/All)
Enter Load Identifier:
[This is the name of the operating system. Type Windows XP Professional or Windows XP Home Edition.]
Enter OS Load options:
[Leave this field blank, and then press ENTER].
After you perform the preceding steps, restart the computer, and then select the first item on the boot menu. This should allow Windows XP to start normally.