mepal, if you have a Toshiba this will work (it may also work for others but I couldn’t say). Hit fn+tab. This should cause the fn menu to pop up. When it does, close it and fn won’t work.
If you never want to see fn again, you can edit your startup menu. The simple way to do this is if you see the fn exe program in Start -> Programs -> Startup. If you don’t see it, then you can do Start -> Run and type msconfig.exe. This will give you edit access to config.sys, which is probably something you shouldn’t try unless you have confidence in your ability.