bde connect to oracle8i error?

I connect Oracle8i(8.1.5) with delphi5.0 ,but there have an error :

 General SQL error
 ORA-01041: internal error. hostdef extension doesn't exist

BDE using
using oracle Sql&plus can connect oracle8i.

I have check my path ,there have ora8i\bin.
I have it maybe this problem:
   BDE can not find oracle TNSNAME.ORA file, because when i config BDE,
click "server name " parameters item,there will get a error "bad file name".
    we I install oracle7.3.4 client first, BDE can run. then I delete
oracle73 path ,then I install oracle8i client in another flod. and config
oracle8i. I find server name list is :what I configured using oracle7.3 , If
I copy oracle8i\network covered ora7.3's  network\,  then I find this time
configure is correct. and DBE can connect oracle8i.
   if I install oracle8i client  in another computer where oracle have been
remove, the problem come again.
  So I think when I install oracle8i, delphi bde cannot find TNSNAMES.ORA

What can I do to fix this?