Ivor,
Reinstall FTN95 from your original CD, or from the files you downloaded when you got your licence, and re-input your original license info when it asks for it. That will probably make the message go away. Unless you specifically need facilities in a later version of the compiler (consult bug fix list on the website here), being one or two releases back doesn't hurt.
You can, if necessary, use the Personal Edition salflibc.dll with a licensed compiler without causing the message to come up, which means that you do get the benefit of some bug fixes! The later versions of ftn95.chm (the online helpfile) are much larger, generally nicer and more accurate than obsolete ones. Hence it might possibly be worth saving those files from the appropriate folder and copy them back after the install is done.
Eddie