password for postgres

i don't know y am i getting this problem

when i try to start off postgres
it asks me for password:

i did not set any password as such

except that when the first day i used template, i ahve used the below
two statements
ALTER USER postgres with encrypted password 'your_password';
ALTER USER postgres with encrypted password 'welcome';

but it is not accepting both the passwords
i am getting incorrect password after three trials it is returning back
to command prompt

when i have used
select * from pg_shadow;
then i got

md5d31faa0b92fad4e2d8e4af34a30f890b

though i use this i am not able to acess i don't know what to do with
this issue
can any one shed light on me by explaining me what was the mistake i did
or which password to use
thanks for any help

Regards
kusuma.p