Sorry i'm late in the game with this, but it does not appear that this has been answered successfully. You do not, i repeat, do not want to use fdisk /mbr with Windows NT or XP. That method is for dos based systems and not NT based systems. You swill most likely have success with the following:
fixboot c:
fixmbr
is you want to change the boot.ini file then execute the command:
bootcfg /rebuild
the last will put a new entry in the boot.ini file.
To do this successfully you should have started your system from the Windows XP CD and selected the 'repair' option.
This also assumes that you have not moved any partitions around, that is in the order or number that they exist on your HDD. If you have then you should execute bootcfg and rewrite the boot.ini file.
You also should be in the root directory, as in 'C:\' not 'C:\windows'.
Please don't ask me why I know this so well. As most people in our industry know so well, sometimes you lear VERY well by experience.
Hope this helps others.
John