Get your Windows XP CD CD and boot to it, when you get to the Setup Screen, press R to go into Repair mode.
After this has loaded, select the drive you have Windows installed on, by default this is usually '1'.
Type in your Administrator password (if any)
Now you want to replace your missing or corrupt bootloader, type in these commands where "X:\" is the letter of your CD Drive
COPY X:\i386\NTLDR C:
COPY X:\i386\NTDETECT.COM C:
After the files have been copied type in FIXMBR, when it asks you what to do, press Y then enter.
Type EXIT and remove the XP CD from the drive, Windows should now load.