I had another go and think I can offer something extra people can do to make sure this works. It seems to have worked for me.
Just in case people don't do this as well because it might not be entirely clear for some people: When uninstalling FH+, use the game's uninstaller. Do not just delete the programs folder or use windows add/remove.
Once you've uninstalled, double-check that the program folder is gone. Then use a cleaning program like CrapCleaner to get rid of any temporary files that might be lurking.
You are now ready to do a clean install. Follow Sameth's instructions, and make sure when you name the directory you use all letters. In other words, don't name the directory FH+. Name it FH or FHPLUS, doesn't matter as long as it is all letters.
Like I said, I had problems on all 3 pcs I tried it on, but as soon as I had used CCleaner after the uninstall, they all started working.