The support for relative paths for Ant scripts is wrong.
When using i.e. the target
-buildfile ..\..\..\..\..\MyBuildFile.xml
The Hudson server uses the folder
%HUDSON_HOME%\jobs\MyJob\builds\workspace
as the working directory
If Hudson finds the MyBuildFile.xml, it passes value for the
parameter "buildfile" to Ant, using a cmd.exe.
cmd.exe uses "%SystemRoot%\System32" (C:\Windows\System32) as it's working
directory.