Someone (thanks Bruce) has alerted me to a mistake in Portable GIS, which luckily requires a very minor patch- so minor that I am not going to release a new version now and force everyone to download it again, although it will be fixed in the next version. Basically there is a line in the apache config that doesn’t update it’s drive letter because the slashes are the wrong way around.
Go into .\usbgis\apps\xampplite\apache\conf\httpd.conf with a text editor and find the line (should be right at the end of the file):
SetEnv PROJ_LIB N:\usbgis\apps\FWTools2.1.0\proj_lib
Where you might have a slightly different drive letter. To fix, change the directions of the slashes in the file path from \ to / and change the drive letter to whatever your current drive letter is, and save the file. Then when you change the drive letter in future and run the xampp setup, it should automatically update.
Thanks for your patience, thanks again to Bruce for pointing it out, and thanks to everyone for downloading it!