How did you install to a USB?
This from the readme-en.txt file and often overlooked:
* QUICK INSTALLATION:
[NOTE: Unpack the package to your USB stick or a partition of your choice.
There it must be on the highest level like E:\ or W:\. It will
build E:\xampp or W:\xampp or something like this. Please do not use the "setup_xampp.bat" for an USB stick installation!]
Running the bat file will create paths in your conf and ini files which may conflict your actual USB paths.
Your files are best placed in a folder within the htdocs root folder to save confusion and to keep all your own files in the one path location:
with included files within the yourfiles/commonfiles folder so the path to the includes should be found.
Using the xampp folder to put your own files is not recommended - also a URI would be http://localhost/xampp/index.shtml
for files in the xampp folder, best leave the xampp folder for exclusive use of XAMPP files.
This thread although related to DreamWeaver has some tips on include paths for php.
The php.ini file mentioned is in the apache\bin folder
See how that goes.