Extensions are not loading

I am using since several weeks pFF, which is loaded on a USB-drive, and I am encountering some problems with it. On some PC (regardless of operating system Win2000 or WinXP) when I start the programm the extentions are nor loading. Checking the extention menue it says that these will be installed when I will restart pFF. On other PC the extensions work perfectly well. Does anyone have an explanation for this behaviour?? What can I do (check, change) when the extensions fail to load???
I appreciate any help very much.