Some troubles from Postgrres_beta4

Dear Administrator,
My name is Do, live in Vietnam.
I am a beginner to learn PostgreSQL Database.  I have
 installed Postgres_beta4 for Windows 2000 Advanced
Server, and it is working very good several days (a week).
I have created database, table, etc. Everything I have
done very good.  Until now, when I start "pg_ctl", it give
me a massege "postmaster successfully started", but when I
connect to server in command :"psql -d mydb", it give me a
message  "psql: server closed the connection unexpectedly
This probably means the server terminated abnormally
 before or while processing the request."
Please help me dealing with this problem.
Thank you in advanced.
Ngo Cong Do