Are you using MDT 2013 with ADK 8.1 to try and migrate Windows XP to Windows 7? Then chances are that you have experienced a problem with the bootsect program or the presence of the bootmgr file.
The bootmgr file in C:\ should not exist on Windows XP, and MDT will try to use it if it exists. MDT creates this file once you try to deploy your MDT task sequence and thats OK, but it should not exist prior to running your MDT task sequence.
Second, the bootsect program used from ADK 8.1 and is located in your deployment share folder under tools\<architecture>\ can produce the wonderful error message on your Windows XP machine, which says it is not a valid Win32 application. Unfortunately, this is true, and Windows XP will therefor be unable to use it to set the boot sector on the machine you wish to deploy to boot using your Windows PE installation in MDT.
The only solution I have found for this error is to the use the bootsect program from the Windows ADK version 8 instead of 8.1. Install the PE environment and Deployment tools from version 8 (on a separate machine from your MDT host) and extract the bootsect programs for both x86 and x64, and replace your 8.1 versions 8(on your MDT host) with the ones from version 8. Do backup the 8.1 bootsect programs first though.
If you simply want to test it, replace the bootsect programs in your deployment share under tools\<architecture>\ and try your task sequence again. The reason for replacing the files in the ADK 8.1 install directory is because MDT will use those bootsect programs every time you update (regenerate) your deployment share. If you have not replaced the files, it will use the 8.1 version and your task sequence will once again fail.
Some mention different using Windows PE versions, but for me it works with version 5 from HP, since I deploy HP machines.