Retore Postgres DB without dump

Hello all,
I had a look at your last year archive without finding my solution, but
I suppose it's a common problem.
I am trying to restore an old database from their relations files, I
don't have any dump, is it possible and how ?

System information:
Original - db: Postgres v.7.4, OS: Linux Mandriva 2006
Current - db: Postgres v.8.1.11, OS: Suse SLES10sp1

What I have tried is :
- To copy the files directly in an empyty db, I got a message like this
when trying to connect: "Error: wrong files version, these are 7.4 files
version and you are running 8.1"
- So, I have build and installed a 7.4 version of Postgres
- Copy the files in an empty database
- Connection was ok, but "\dt" told me "no relations found".

What can I try next ?
I also have the original "pg_clog" and "pg_xlog" directories, and I have
also tried to replace them. What can I do with the WAL segments ?

Thanks for your help,
Philippe