http://www.michaelstevenstech.com/XPrepairinstall.htm
Слагате инсталационния диск на WindowsXP, влизате в "To repair a Windows XP installation using the Recovery Console, press R"

FIXMBR C:
FIXBOOT C:
COPY 'CDDrive':\I386\NTLDR C:\
COPY 'CDDrive':\I386\NTDETECT.COM C:\
ATTRIB -H -R -S C:\boot.ini
DEL C:\boot.ini
BootCfg /Rebuild

Опциите: /fastdetect /NoExecute=OptIn

Reboot