> Do you know how to fix the database for taking out these NAN values?
>
> I did gFix several times, gBak and restore but it is not working for this.

The database is not physically corrupted, it just contains values you do not want,
so gfix nor gbak wont help.

> Is there any way for instantiating the records that contain the NAN value?
>
> I have tried to make a SELECT... WHERE myfield = NAN but doesn't work.

You can use this crazy query (I tried only with FB2.5, where it works!):

SELECT ... WHERE myfield <> myfield

Ivan
http://www.volny.cz/iprenosil/interbase/